I agree that the music genre so far, and excluding Wii Music (tho I hated that game), has been very much the same. It hasn't been around long enough for us to see if it can become something as diverse as other classic video game genres.
That being said I can see the FPS genre starting to have an offshoot that is indeed as shallow and niche as the current music genre. And that is the stereotypical realistically based war games, like Call of Duty or Medal of Honor. I thiink they are going down the same path.
I will not say the same for something like the Halo franchise. Halo is a game that, because of it's initial decision to not be based in reality, gives it more leeway in terms of creative ideas for gameplay. And that is what it is all about. Gameplay.
If all the other side elements are too restricting, then the most vital element of a game will become restrictive as well, from a franchise based perspective (being able to improve upon in sequels etc...).
Seriously. Halo can do a lot of things that don't make sense for a war game because it is ALLOWED to. Why? Because the creators said so - they didn't base the game in strict reality. This applies to shooters, but also all other genres who are based too much in reality
