Guitar Hero series will come after Guitar Hero IV (World Tour)'s holiday release. This follows the pattern of Guitar Hero [X] and Guitar Hero [Subtitle] that we've seen since Guitar Hero II's release in 2006.
There are no other details on the game, but no doubt we'll hear about it sometime this year. The only thing we're curious about is whether Activision has managed to secure exclusive rights to Metallica's music, like they have with Aerosmith. We certainly hope not.