I can only think of one movie - The Empire Strikes Back – that obviously had to have a sequel. Even Star Wars could have run on its own just fine.
Obviously it takes a bit of guts to make a movie that needs a sequel, since if the initial movie bombs, then you have an everlasting mark of its failure.
I do remember a movie that left an opening for a sequel but never had one. I can’t remember the name, but it came out in the 80s, and had a kid who played videogames fight against real aliens.
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.
“Remo Williams: The Adventure Begins”
It’s called the Adventure Begins, for God sakes. I am still waiting.
Also, where is History of the World Part II: Jews in Space?
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.
History of the World: Part 1. The title says it all. Where the hell is part 2???
I really hope they don’t it, but the Super Mario Brothers movie ended in a way that suggested it was going to have a sequel, I forget how exactly. But it bombed so I understand why it never happened.
Buckaroo Banzai. No, the movie doesn’t require it, but I want to see Buckaroo against the World Crime League! Damn it, I’ve been waiting all my life!
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!