I think if ANYONE would react poorly to having characters from their IP's used it'd be Nintendo. Otherwise I don't think any fighting game company would care too much so long as you're not making money off of it.
@SeanAltly...if it were Richter Belmont I'd crap my pants. Not that it is but if it was oh hohoho I'd be excited as all hell. I'd love to see your take on him.
It's a cool idea as long as the sprites all are in the similar style. Any adjustments you feel are needed for Akuma are cool with me. Are there any plans to have a Mortal Kombat-esque stage included?
What do you mean by "The last Big Bangs for Mugen"? Also I haven't seen anything on Richter for a while now unless there's something goin on I don't know about.
Ermac and Reptile are in need of new ideas but they will happen. Also I wouldn't use Chucho's They aren't quite up to par with what would be needed to match Sean's style. This is all quite theoretical though. Sean's game is good as is though.