Really?! Sweet! Might I expect there to be a boss, mini-boss, or special encounter with a girl that has a bellybutton fetish? That'd be awesome. :cool
I like the boss rush idea, but Drider is a mini-boss, right? So why not have a mini-boss (or two) attached to each boss? That way there could be more fights without needing an extra grid. However, you could have multiple grids. A boss grid, mini-boss grid, and maybe a special encounter grid.
I agree with marcelostalker. The whole boss rush grid reminds me a lot of megaman.
EDIT: BTW, are you thinking about putting in a button scene (mid-battle or otherwise) that will rival Tower of Gargalesis's Lady Venus mid-battle scene? No one has been able to top it yet, or really even get close.