It's hard to say it's the next big genre when it's already big. GTA was one of the biggest games of the PS2 era. Batman, AC and others already use it to some extent today. Open world is a lot of fun because it makes you feel like you are truly in control as a gamer. I think more games will incorporate it. Expect the next Zelda to be very open world, as well as Monolith's X. Watch_Dogs and Detiny will be too. So you will see open-world rpgs, open-world adventures, open-world platformers and open-world FPS. An open world 3D Mario could be pretty cool.
I guess I wouldn't call open-world (or Sandbox) games to be the next big genre, but I would say that most genres will incorporate open-world elements because it is fun, it sells well and it is becoming technically easier.
