A Knight’s Tale. I suppose it had the happy ever after ending, but there are SO many directions it could have gone in. Adimar told William that he had entered into negotiations with Jocelyn’s father for her… Just because William was made a knight, that does not mean those negotiations would have stopped.
Would have loved to seen more of my beloved characters.
This, and Unfortunate Events. I do think I would have thoroughly liked Golden Compass had they made more, whereas for ASoUE I would have enjoyed it only for the visuals.
Serenity
Definitely Buckaroo Banzai
Princess Bride
When Worlds Collide
Although I don’t think I’d like them filming the Wylie and Balmer novel After Worlds Collide
Forbidden Planet actually did have a sorta sequel, a much lower-budget black and white The Invisible Boy, but I’d like to see a real sequel.
Similarly, they gave Schoedsack and Cooper less than half the budget and only nine months to film the sequel to King Kong, so they came up with the overly padded and comic-themed Son of Kong. But they coulda done much better given enough time and money. I’d love to see that.
Reservoir Dogs needs a sequel with all the surviving characters: The Sam Jackson lookalike, the waitress, and the Silver Surfer poster.
“The Incredibles” is definitely at the top of the list for me.
Other movies? Any super-hero type of movie could go with a sequel, like “Mystery Men 2” or “Another Undercover Brother” or “Bigger Trouble in Smaller China”.
On a side note: When I was a kid, I used to joke with my brother that the least essential sequel would be “Micki + Maude 2”. But somehow fantasy has merged with reality in my brain and sometimes I’m not sure whether “Micki + Maude 2” exists or not! So maybe they should really make a sequel to “Micki + Maude” to put me out of my misery.
I want a third installment, perhaps a reboot, of Ghostbusters.
I want a sequel of The Goonies, with all of the original cast.
Mostly to satisfy my inner child.
Unbreakable. No twist, just gritty heroics.
Fight Club. That ending was so interesting.
The Incredibles.
I’ve always wanted to see “Charlie and the Great Glass Elevator” get made, but I don’t want Tim Burton or Johnny Depp to have anything to do with it.
Definitely Series of Unfortunate Events. Not only do the books call for it, but I thought the movie was pretty good. Underrated.
Ghostbusters 3 is coming…eventually. It was supposed to be out this year, but filming got pushed back to fall of next year, unfortunately. 
I always wanted a sequel to Will Smith’s Wild Wild West. Also would love to see one for Evolution, Slither, and Night of the Comet. (Yea, I love cheesy movies.)
I am a sucker for post apocalyptic stuff, so I have always wanted to see sequels to stuff set in them to see how society starts to rebuild itself.
What happened after The Day After?
How does society develop after The Stand was over?
Though to answer the OP, I would love an Equilibrium sequel, as well as a decent sequel to The Last Starfighter. Lot of meat on the Starfighter bones, imo…
still waiting for Escape From Earth
There still isn’t an agreed upon script. Murray is in as much as he is out. It’s probably not going to happen.
The reboot is more likely. I’d have been happy if they had done GB3 in the late 90’s and used the original concept of Ghost Busters as municpal workers like garbage men. Sandler, Farley, Rock and Spaid would have been great.
Would absolutely love to see Gillermo Del Toro do a sequel to Hellboy. No really - I liked the second one. Maybe some backstory into how Johann Kraus got that way, and a revisit from the Death character where Liz really does experience the consequences of the deal she made. I would want to the entire movie to be like that “Toro-esque” scene with the Death character, and dial back the slapstick.
Sandler in my beloved Ghostbusters?
o thank you merciful God for not allowing that to happen
Master and Commander. I’m a big fan of the books, and I thought the movie captured the spirit pretty well. I’d love to see one with some of Stephen Maturin’s spy stuff included.
I forgot about Ferris Beuller. A sequel to that, adult or teen, could have been pretty good!
I’m very wary of suggesting sequels to movies I love because they so often suck, but if I had to pick one (other than the already mentioned Buckaroo Banzai) is would probably be Ronin, but only if the right director and writer(s) were involved, and both Robert De Niro and Jean Reno reprised their roles.
There was a Breakin 2