uber-difficult mode support. Like, people play pokemon with only 1 pokemon, or only use white mages on final fantasy tactics. Or not using GFs in FF8. Just support, not necessarily a game mode.
a story.
AI player, like a coop mode.
joystick support? like you did with coptra?
if you can, what about 2 player mode? (2 joysticks, or 1 mouse 1 joy)
Music! Multiplayer would be nice maybe, not necessary though. Definitely need more arena, and an infinite one that's just like you start in it lvl 1 and just see how long you can survive and if you live past a certain wave you can choose a perk, class, or weapon. And a super difficult mode would be nice.