Films that deserved a sequel but didn't get one

[JEM]

Highlander is a classic example of why you should not make sequels.

Highlander II & III were absolute shite.

Star Wars is probably one of the few exceptions as the nine episodes had already been written.

I'm delving into nerd territory here but the Star Wars films hadn't all been written - Lucas wrote them all picture by picture, he just had very, very sketchy ideas about them en masse. He didn't know which characters were going to appear in what films or which direction the films themselves were going to go in, other than the very basic plotlines leading up to A New Hope.