Well, first off, the hidden blade HAS to be used (if anyone has beaten the game, you'll know what I mean).
Secondly, I'd imagine that it's possible that guns could be taken from gameplay for one reason and one reason only: the Pieces of Eden. The Templars have the Pieces of Eden and as we've seen with the Apple, they can be used to hypnotize. At the end of AC, they hinted at using the Apple to hypnotize people on a mass scale. Why not use it as a reason to get rid of guns? It would take away a HUGE threat to them at large. This would leave the assassins with close range weapons as their only choice. I mean, the Templars could always use guns themselves but doesn't mean that the assassins need to. I dunno, just thought I'd throw that out there.
My last idea, the gameplay doesn't need to be in large cities either. Beat AC2 and you'll understand. If anyone wants me to elaborate on this idea more, just let me know. I just don't want to give away too much plot for those who haven't beaten it yet.