Pixel graphics and balanced mechanics really sold me, so anything else is extra. I feel the best thing about the game is the very well crafted pixel graphics. Second is the elements, it feels fun to mix them and such. Reminds me of magic the gathering.
I'm interested in rpg elements myself, critical hits, or towers having a kill count where random stats increase depending on how many monsters they have defeated.
Or an item shop that has speed/range items to more personalize individual towers would be cool and add a ton of replayability.