Making an Amiga game is a good idea, but already everybody is bogged down in politics without anything having been agreed. Perhaps this is one of the reasons so many projects get shelved, and so many "good ideas" turn out to be utter crap in the end. Too many cooks spoil the broth. If this isn't going to be a commercial project, rather just a showcase of the Amiga's talents, there are two points to consider. (IMHO)
1) It doesn't matter what the subject is... any game will do, but...
2) It needs to be done in a way that pushes the hardware to it's limits. Otherwise it's just going to be an average "wannabe".
You could write a game of Pong, as long as it looks and sounds impressive, at least better than any other pong game. (silly example, but you know what I mean!)
If you want people to say "Wow, an Amiga can do that?", you might as well write a demo... that's what demo's are for.
If you want a game that people will play and play again, it needs playability (which doesn't require good graphics or sound) AND it needs exceptional graphics and sound... because virtually every type of game already has lots of clones or derivatives that feature one or both of the above in varying amounts.
I'm in now way artistic graphically, and my coding is both high level and crap, but I really hope that Akira gets this off the ground and shows that it can be done. So Akira, this is your project... choose an idea and stick to your guns. Don't deviate from that path until it's done. If you then don't like it... go back and redo bits of it, but make sure you don't get pulled in all directions by other people's "good ideas" (You'll end up with a big mess if you do). I'm sure you'll find people who will help you create your project, even if they think you should be doing another Doom clone instead.
By the way, I'd REALLY like to see 'Serious Sam' on the Amiga... even my little 33Mhz Sony Clie has a version!
Good luck and keep us posted.