How about a real achievement system (aside from skins)?
Something like "Achievement: Electrician (Do 1/10/100 million damage with Chain Zapping)" // "Achievement: Ninja (Get 100 killshots with daggers)" // "Achievment: I have seen that before (re-run the same arena 20 times without a rebirth)"
Could add a "x2 income every 22 achievements" mechanic.
You have a ton of possibilities here to channel your creativity into, and should you ever get it to a state where you can realistically put this on Steam you'd already have a tie-in to that achievement system ;)