Warren Spector, the chap behind Disney's Epic Mickey has ideas for at least two sequels depending on how the first turns out.
The Wii project tells the story of ancient Disney characters isolated in a darker, alternative world and just as hope seems to be lost, Mickey Mouse turns up to paint a little colour. If it works out then Spector already has at least two more planned out, but it all comes down to how well Epic Mickey's debut does.
I don't do anything that isn't extensible. And I will certainly feel like that I have not done my job if we can't make other games in this world with these characters. In my head, I've got two more planned. In the business world of reality, those games have not approved, and who knows if we'll ever see them. I had three games planned for Deus Ex, and you see where that got me.