I loved IX, although it would be nice to see a sequel, there is no need for it. The game had a (SPOILER)happy ending and its conclusion is fine as it is, it doesn't need to be dragged out.
There could be a sequel based on a member of IX that isn't very explored throughout the game, such as Beatrix as somebody earlier mentioned. Because this wouldn't ruin IX just like they ruined X with X-2.
Personally I'd be much happier with a remake of the game with PS3 Graphics. :rolleyes2