If you’re thinking of The Last Starfighter, it really did wrap things up nicely. Yeah, the chief bad guy bailed out a la Darth Vader, but there wasn’t anything about the ending that screamed “I need closure”.
Of course, the first two installments of the upcoming The Lord of the Rings trilogy will both require the closing act (which the Bakshi version did without, much to the detriment of its reputation by some Tokien fans)
Also, Back to the Future 2 was more of a transition piece between the first and third and, IIRC, left some loose ends needing to be tied up.
Definitely Bill and Ted’s Excellent Adventure- for how else would we find out how Wild Stallyns’ music reaches the world and how our heroes become the basis for civilization as we know it and all that, but in Bill and Ted’s Bogus Journey?
Can’t think of anything else. Mmmmmmmm. Nothing else I can think of that really begged for a sequel.
Why The Godfather, of course. You had to continue with the story of Don Michael Corleone. The third one, while OK, wasn’t really needed. If only Sophia Coppola was a better actress!! You know who was originally going to play Mary? Winona Ryder! She would have been much better! A stuffed teddy bear would have been much better!