Banjo-Kazooie was released in stores just last week, but its sequel, called Banjo-Tooie, is already under way at Rare. Not much is known about the title except that it will expand on the original game in some way. Theoretically, Rare could be developing a 64DD sequel disk, which could include a full sequel and a "second quest" type game so you could go back into the original and play through again with a somewhat new game.
As stated on Rare's home page: "We've included some areas in Banjo-Kazooie that can't be accessed without first playing Banjo-Tooie. The secrets revealed at the end of the first game will only come into play during the second, so until then, you can look but you can't touch."
Banjo-Tooie will be released in 1999.