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.