Pokemon, what happened to you?

[Vipershark]

well... it's obvious

same gym leaders (except gary is the 8th gym), same elite 4 (well, a couple of different ones, and koga is elite 4, for example) the characters look more mature (misty, for example), and the fact that you fight Red, the hero of the previous game who is now a pokemon master at this point.

I'm not going to do a search for official word on it, but it's an obvious sequel.