Well, they've already made 3 (4 if you count GH3) and 5 if you count rock band. Judging by popular demand, I'll say guitar hero makes it to 5 and rock band makes it to 3, both before the 2010.
Good bye everyone, it's been fun!
Special thanks to ED_, superman4u, and Cross Stinger
Credit for the Siggy goes to shadow slayer