I'm not suggesting an online multiplayer mode, but maybe you could have a co-op mode with someone next to you. Then there could be a separate co-op story mode! Or, the classic two player "swarm" arena-esk mode.
Is it just me, or would other people like an actual map that we can open aside from the mini-map? I know you're probably going for that "discovery" sort of feel, so maybe in order to counteract the map, you could make it so we can only see the parts of the map that we've been to already.
The most obvious need for a game like this is for the music and the game to be in sync. This game does just that. Not to mention the music being awesome just by itself. I mean, Epic music and a bad game = no. Bad music and a awesome game = acceptable. Epic music and awesome gameplay = Bullet Audyssey. How does this not have a average rating of above 4/5 stars ?!?