I would love to see the sequel to Hunchback of Notre Dame redone. I would keep Madelline but make the story a lot more dramatic and less cheesy. The animation would definitely be bumped up a few notches and Sarousch would be much more sinister.
Personally, I think Hunchback of Notre Dame 2 was totally unnecessary. The first movie ended on a perfect note, and by making a sequel they only cheapened its message: that you don't need to get the girl to find happiness.
Now one movie I would love to see a sequel to is Oliver & Company. At least that movie had an ending that left things open for future stories to be told. I would love to see some new characters like an adult cat that befriends Oliver, some exploration into Jenny and her relationship with her parents, and a new villain that's just as, if not more threatening than Sykes. (Maybe make him as bad as Frollo)
It wasn't the worst sequel (I reserve that "honor" for Mulan 2) but it had nowhere near as much heart and soul as the original.
That's the thing that really bugs me about the sequels: they all have potential to be good, but the people try to push them out so fast that the quality really gets knocked down a lot of pegs.
I think Bambi 2 is the all-time best because that one was made with more time, love, and money, and it actually contributed to the original movie.