The Saint. This has the potential to be great, with or without Val Kilmer. Perhaps a prequel?
A sequel to THX 1138 might me interesting.
I know they've been trying for another Ghostbusters.
Kill Bill Vol. 3 After Vernita Greens daughter grows up, she seeks revenge? And Elle...?
District 9 might be good. Did I hear that they are doing a sequel?
The worst sequels would be:
Titanic part 2 (I know, they already did that. and it sucked!)
Groundhog Day. Perfect as is.
Same goes for the Back to the Future trilogy.
And, despite Ben Stiller's insistence, Lord of the Rings. (Hobbit doesn't count)
Manos: The Hands of Fate. Although a sequel is pretty much guaranteed to be better than the original. I don't think it's possible to make a worse movie.