The longer virtual worlds exist, the more that developers will see that it is better to evolve than re-engineer. I would be very surprised by a sequel to WoW, there doesn't seem to be any point. Everquest II showed that it was probably unwise. Sony would have been better off devoting that time to a EQ overhaul or just work on a new IP.

So over time this gen, will become next gen. When people were talking about the next 'third gen MMO', what they didn't realize is that they were probably already playing it, it just hadn't evolved by then.

The last change in MMO generations was the introduction of virtual environments and social areas ( a la Facebook, Second Life etc), just hanging out in an environment is enough, without playing a game. This is getting fed back into the mainstream game releases, with the addition of offline content, social linking of avatars etc.

The next 'gen' will be greater sensory immersion, the game beyond just a monitor. If Blizzard are not already working on this, I'd be very surprised.

Blizzard don't have to make follow ups as the market leader, they are in a position they have to innovate to protect their market share, otherwise it will be slowly eroded over time by all the other releases.