I feel like some kind of upgrades for monsters would be cool, maybe evolutions/mutations or something like that, i also had an idea about monster like slimes which if you have enough will merge into one more powerful slime, or something like combo, where if you have certain number and type of monsters you will get bonuses, this will bring more tactic and deck building aspect into the game, maybe some random events, or events that we could buy for money, or for new currency like mana, for example: event - graveyard, every third death will spawn skeleton, cost 2-3 coins(i think new mana stat would work better with this).
maybe more boss-like monsters, which are much stronger, have many abilities but are expensive as hell, there can be only limited amount of them, maybe in order to summon them you have to meet some requirements or something along this lines.
this game has huge potential, I'm in love with the concept and I want this game to success, huge props to developer/s ;)