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.