hear me out, not sure if you're planning on adding new content. but an endless mode overhaul, with endless mode exclusives. and after you beat a level, you choose one of 3 modifiers to add. maybe even add really rare positive modifiers! like "You are faster" or "You are hardier" or "you are shielded"
(of course you'd be able to play old endless mode with like a cheat code or whatever)
I also think this game would be perfect for steam. I'd buy it.