Yeah I'm probably just going to put that under the "special secrets for each level that you unlock after getting all badges" category, for now. But who knows. I'll consider it, I just don't think I'm very good at writing good relationships.
How about something like, getting all of the badges in one stage lets you do the endless mode as truly endless without having to worry about getting hit once?
Not sure if this is what the other person meant, but I'd love to see a stage with the same girl, but in a different pose and/or different outfit as a reward. Like, something less casual, like a dress, would fit for a "date" setting.