I like the bayonet idea. By the way, just going off of memory, I think bayonets would reduce accuracy. That was back in the days of the musket, though, so it might be different now. I also like the grip/padding idea, and the different bullet types. I could live without color changing, but it would be pretty cool. Grenade launcher would be... interesting. I think everyone would just complain about it.
This idea's been thrown around a lot, but how about, instead of a scope, you can purchase iron sights (I think that's what they're called) for your guns? And a barrel mag for assault rifles would be insane, but hard to implement.