That's a great film. It was only a moderate box-office success though, but I agree that it would be fantastic to see the four main actors (i.e. Kevin Costner, Kevin Kline, Scott Glenn and Danny Glover) reunited. That said, are Kline and Scott still working? You hardly see them in stuff these days. I'd also try to find a way to bring back Goldblum. I forget how he was killed in the film, but maybe his character survived somehow.
Devil's Advocate
The Long Kiss Goodnight
Misery
Primal Fear
My Best Friend's Wedding (This could be a fun sequel though catch up on the couple and the jealous friend)
Pretty Woman
Con Air (I don't think they could pull this off anymore)
Almost Famous (Although an 80's sequel could be fun I think they would wreck it)
There are a lot of them if you go back 40 years (1983), probably hundreds. Just to name a few (some may have already been mentioned):
The Terminator
Predator
Commando
Jurassic Park
Lethal Weapon
Beverly Hills Cop
Home Alone
Goodfellas
Unforgiven
Beetlejuice
Titanic
Napoleon Dynamite
Office Space
RoboCop
The Princess Bride
Top Gun
WarGames
Ghostbusters
Revenge of the Nerds
The Karate Kid
Young Guns
Lost Boys
Die Hard
A Nightmare on Elm Street
Dirty Dancing
Platoon
Full Metal Jacket
Scarface
Rain Man
Driving Miss Daisy
Braveheart
The Breakfast Club
Fatal Attraction
Pulp Fiction
Kill Bill
The Silence of the Lambs
Fargo
Saving Private Ryan
The Usual Suspects
The Sixth Sense
Fight Club
The Matrix