I think Corey may have convinced me about GB3. But i'd prefer it to be live action, it would be funnier that way, just to see Akroyd and Ramis trying to run around seeing how fat they are.
As for the soundtrack, considering Run DMC did the theme to the second one, I could see another rap song to accompany this film. Either that or a dance remix of the Ray Parker orginal. To be honest, as long as they don't get Green Day or MCR to do a "punk cover" of the orginal (ala the Simpsons movie) I'd be happy.