Okay, having had more time to think, and you asking about deck types that are different but have depth:

I think you could get really far with a bunch of dudes who "Eat mana", in the sense of "When you target me with a spell, I get +1/+1"   "When you target me with a spell, I create a minion"  "When you target me with a spell I get a free attack" and the like.

Then, replace the various curse/summon/protection spells with ONLY spells which target your dudes. Maybe a healing spell? MAybe a "+1/+1 bonus" spell, maybe a teleport spell that swaps two of your units... or maybe a "Manaflare" spell which doesn't target anyone, but activates ALL "mana eater" abilities (or... activates an entire row at any rate).

It makes it a little less "Auto", but I think the monster-spell back and forth could work in a cool and interesting way much like the Bard/summoner back and forth.

Maybe even have a monster who refreshes a spellbock on death (If in row one, is book one, if in row three is book three).

The other thing I can image is having monsters that get boosted from deaths in their row. In a like "On kill enemy, get +1/+1" (Great vs sneaks),  Or "If an enemy dies in this row, deal one damage to the backmost enemy"

And... in terms of other ideas:
Traps.
Like, things that you put in your backmost column, and then, if an enemy reaches them, SOMETHING happens (Maybe 5 damage to the first enemy to reach them? Maybe 1 damage to the whole row if an enemy reaches them? Maybe summon charms first ally to meet them, who knows)

one easy way to make decks feel different is to add exclusive mechanics. look at the wildfrost tribes- stuff like the clunkmasters junk/bom/scrap, the snowdwellers shell/snow/shroom, and the shademancers teeth/sacrifice/overburn all make for valid builds in of themselves. Artificial exclusiveness isn't necessarily bad- like a pig unit that only buffs other pigs, but does it really well

If you wanna make different characters, but still keep the multitude of strategies, then consider messing with the core rules. Maybe the "working class monsters" are cheaper but worse, or maybe they require a salary every round/every few rounds.

You could also expand on the character themes you already have, move the ratmen and pig to a beastmaster type guy, and the succubus to a cultist summoner type guy, so everything at the very least feels thematically different. (I would also personally love if the kobold grabed all of their siblings to make a faction)

player theme idea-wise, i have the following: Count dracula. Pharaoh. Living tombstone that uses ghosts.

enemy theme idea-wise, i have the following: Army general. The pope. Shadow dictator Santa Claus.

count dracula vague ideas: Changing lanes. More units in one lane than the others. bonus whenever a specific unit defeats an enemy. Shuffling around opponents. More units of one level/tier than the others.

pharaoh vague ideas: Summoning bad units for the opponent. Giving curses to the opponent. Debuffing the opponent's stats. Throwing bricks and sand and stuff. Some sort of beetle nonsense.

Living tombstone vague ideas: Benefits from rerolling. Benefits from freezing. Benefits from leveling. Benefits from buying and selling. Benefits from food. Putting stuff on tiles. Friendship.

Adding more explosions never hurts i think

What if you kind of combined both, where you play as one of the king’s sons sending peasants to take back the throne, so you use the units used by the king while fighting against the necromancer

Okay, so, first off: thank you so much for such an amazing game. I love it, it is special. I am still trying to get a 6 turn win with the new updates.

In terms of ideas for decks: I reckon a frankenstien deck to go with the necromancer deck (Zombies instead of skeletons. Tech instead of magic. Lighting rods and shit, little green/turquise hearts instead of pink ones?). I can also imagine a "Shadow summoner" deck, or a ghost deck.  (Yes, they are all undead, but hell, the whole cutesy/spooky you have here is a VIBE)

In terms of gameplay ideas... I am going to spitball a million billion ideas, none of which are particularly thought out, and maybe one of them will be useful.

I really do believe that a teleport spell would be amazing gameplay.

In terms of deck archetypes, I think the main thing with the skeleton deck is that there is very low interaction BETWEEN rows.

I can imagine having a monster/effect which boosts everything in a particular COLUMN, and playing on inter-row interaction. That or monsters which like... zoop around between rows somehow? Or monsters that can ONLY be placed in the top row (Zepplin?) or have some other restriction of that kind?  Basically, something which breaks the symettry between rows, or makes rows more interactive with one another.

I think you could build a deck around this, without having it be "Single strategy".

I can imagine having a deck with monsters which take Two spaces (Horses?) Not sure of gameplay, or if is possible in your engine, but is a possible design space.

I think having monsters that explode for damage on death is  a mechanic you haven't used, which could belong somewhere... or a monster which is bad in combat but deals bonus damage to the castle. Or bad in combat, but steals gold from the castle. Basically "On castle" effects seems like an area which you could use, but probably not the basis for a deck.

(Maybe even "On hit castle, deal one damage to king?")

Monsters with an On purchase effect could be interesting. As a base example, having some bats with "On purchase, Spy" could work.

I think you could have some strategy (possibly woven into a larger deck) which interacts on spells more.  Possibly with minions such as "On death, refresh one page from this rows spell book" so that you get more spell uses.  Or... alternatively "On kill, harvest souls, gain spell uses"?

Another possibility is... an army which gets to permanently screw with the battlefield? Like, when your spike gremlin dies, it leaves behind spikes which damage enemies this battle, and for many battles to come?

Potentially, having a unit which is undercosted, and overpowers and.... only lasts for one battle. Its there, and then gone. (Is that bad? Is it a newbie trap? Don't know).

Or... units with a different movement rate (Probably double speed). Not sure what the speed does for the game, but its something that could exist.

I kind of like the idea of a unit with zero damage, but lots of health, and then units that blast damage when they are NOT moving (like, units that like to be stalled).

If you wanted to get really different and weird, I guess you could have a Kaiju style deck, where you start with a single monster, and you just strap more and more stuff to the one big monster you have, and then use it to smash everything?  ... Hmmm... that sounds cool, but probably doesn't match the actually gameplay you've got going on so well.

In terms of new enemy's... Maybe a Wizard tower? With mages, and apprentices, and lighting? 

Or a Church/cathedral come down to smite you for being a Necromancer, and you instead of a castle its a big gothic cathedral, and instead of a king its a... big priest?

Whatever you do, I look forward to seeing it, it is all amazing. Thank you thank you thank you.

slay the spire, Night of the Full Moon and others deckbuilding  proves that just because you know what result you want, doesnt mean that there are only one fate combination, but can be many funny ones.

And If you really want to make pure thematic decks, It can also be very tricky, I mean, take the warior hero by exemple:
the player knows  thet he needs to make units fhysically stroger, but how you gonna do this can variate a lot:

you can focus on base heath, or healing, or armor, or on damage and health, or suport, or progressive in-battle growth, or defending the glass cannon with a defensive unit, or groing units mostly with wepons/itens, or with special units that get stronger with numbers. So are you going to bet on many week units because of that special unit? do you prefer bet on the glass cannon + the defender? do you have enough tanks to afford a in-game growth unit? damage with the HP pays the risk? what if there are lots of ranged monsters? are you strong enough to get there with a lot HP?Is It better to use your sacrifice spell to do a big damage with the higth Hp unit or It would be more useful alive?

"Going back to the question,  a deck without many different strategy combinations will suffer that fate pretty quickly. And it's hard to think of ideas to give a deck some "personality" (the "strong" deck, the "summon" deck, the "support" deck, the "magic" deck) without making it feel like you are going for the same strategy every time."

If you want to avoid that fate combination, you can aways add some neltral units, that appear in any deck, also allow, or even make mandatory to combine classes to make your deck.

You could even add a mode that you make your full deck based on in game decisions like this:
tier 1 all neltral/inicial-class, after you get to tier 2 you choose from 2 or 3 differents classes which one do you like to add to your deck, same happens for tier 3 and foward. So you can play with a assassing priest or a mage hunter.

also, you can make some monsters imune to somethings in order to discorage purely focused strategies.

If you want classes, you can start with some of the classics, like:

A warior, who try to win with brute force HP and Dmg, the combos units and spells would be all incressing those status, or the armor, or someting like that. Like double damage, and than double again. Also some kind of protector, who would take all the ranged damage insted of other units. You can replace magic books by bravery buffs.

A priest that has a lot of healing spells and units. also units with regen, or units and spells that cast suport effects, like a protect, an armor, one restoration of the amunition, haste that make unit move faster, ressurrection, a unit/spell that does damage by the health, not the attack. And for last a holy nova for damage, things like that make a good priest... 

A spelcaster/wizard/witch/mage who ... you know... use magic... and magic units too and combos with magic, like someone who gets stronger everytime you use magic, or a unit that gives back 1 use of magic when it dies, or someone that buff in 1 damage the spells that do damage!

A druid, who transform their units into stroger animals, summon animals and use animals for damage, also poison animals and flying animals. You can replace the magic books by whistle or something to call for animal suport.

A hunter, who mostly use ranged units and ranged itens/spells
you could switch the spells, that traget units, for traps, that traget tiles.

A Barbarian who put everything on offence - units with higth attack, gain rage when less that half hp to hit x2 dmg, rage magic, rage item, itens that gives +attack and - hp, bloodthirsty to give some sustain...

A Bard, who essentialy debuff all the enemies/ buff all the allies. Verry weak units, lots of buffs or debuffs...  switch the magics to songs, dances or something. Maybe everyone doing combos with everyone, would be very diferent for sure.

A thief who steal gold, bribe units, debuf foes and conceal units.

An assassin, who has itens that give double attack, stealth, surprise attack, crit, poison hit, death clock... weak units, but cunning, sneaky, and with tricks. Also, you can switch the magics for tricks.

An aprendice/mimic class, who can use units itens and skills from other classes, but not the strogests, maybe the player could choose which of the tier 2 they want as option or something like that.

The better part is: even if the class wasn't verry balanced, It's ok! it would be still funny to play, some are going to be, naturaly, better and easier that others.

bonus! the classes could have some unique trace, like the necromancer who can win even if she loses all the hearts after beating the king, thats because she is the necromance class and only needs to ressurrect herself after all. 

Before I get into more conceptual ideas, if you're expanding the game a few little QoL things would be nice. Perhaps:

-Skip tutorial button

-Mute button

-Turn counter from the management screen

As for new content, here are a bunch of random ideas off the top of my head:

-You could have 'chapters', campaign style, starting with fighting the humans (what you have currently), then moving on to elves, dwarves, orcs/goblins, maybe dragons for a big finale. That would be a ton of work, especially art-wise, but as long as we're just brainstorming :p

-A vampiric unit seems like a pretty obvious choice, something like (2/3)(2/4)(3/4) Heals 1/2/3 health to self on attack.

-For an high-level option, you could make a unit that has an extra-high amount of health, but takes up two adjacent unit spaces. Maybe a giant snake? I'm not sure how exactly you have your code laid out, that could end up being to complicated to try to implement.

-Another pretty basic idea would be a spear-wielder that damages the first two units in front of it instead of just the first. If you want a flashier version, swap the spear for a flamethrower. Something like (2/1)(2/2)(3/4)

-I notice you have a werewolf sketched out up there, maybe it could do something like double its own health and attack after the first time it takes damage? Or after it attacks the first time. Either way, you don't have a unit with a once-per-round buff yet I believe.

-The necromancer's familiar, the hat dude, could show up as a unit under special conditions for a little bonus :3

-Especially once the game starts getting longer, a means of regenerating the player's own health would probably be a good idea. That, or increasing the maximum. In the latter case, you could have some kind of 'bulwark' building to provide the buff.

-For entire other decks, goblins seem like a fun theme. I bet they would look great in your art style too. The underlying idea of 'breeds too fast' would make a nice counterpoint to the undead of the base deck. A theme for a deck with maybe more unlock requirements could be elemental spirits. That would afford a whole lot of freedom with designs too.

-Since not every unit from the 'base' deck is a skeleton, maybe the outliers such as the lizardman and pigman (being high-level units anyway) could be 'mercenaries' that show up in every deck.

-A neat spellbook would be one that can swap two units while in battle. Level 1: Do it once, to your own units. Level 2: Do it twice. Level 3: Do it three times, and you can do it to opponents too.

-I really like how your mechanics for purchasing and upgrading units are designed. I think the weakest aspect is the spellbook slots though. I'm not sure what the best way to fix that would be without swinging too far and making spells overpowered. Maybe spell slots could be their own unique thing separate from unit slots, but the slots unlock at the same time that your cauldron upgrades? I'm not sure.

This is actually a lot of stuff now, so I should probably stop typing lol.

some fusions would be interesting, like a hero that you can follow 3 paths a pure evolution, a fusion evolution with another hero lv2 or a 2nd fusion with an alternative hero.

or a heroe that get a evolution from a item, or from a magic casted on It.

or a heroe that evolves from killing a monster ou some monster

I would love to see
Unique bosses like a big slime that divide itself in 2 or 3 everytime its HP reach a half. A big robot that fire laser bean on center to hit every monster you have, on the top and botton you have to destroi It's arms, after that your units from botton and top that survived can join the mid lane. 
Big bosses that uses all the 3 lanes at the same time, attacking and beeing attacked
Bosses would be Awesome!

I would love to see a world 2 where monters are stroger, more life, more damage. Some of then could summon stronger monsters, or have 1st hit invencibility, or fly! they would be imune to melee damage.
Foes that attack from behind your formation, wizards whose attacks ignore all the armor or invencibility, a troll or something that use a club to do a splash hit. A poison monter, a poisoned big monster.