Movies that promised a sequel ...

[retrorussell]

Buckaroo Bonsai

Apparently there was to be a Karate Kid crossover with that film, where Mr. Miyagi teaches Peter Weller how to care for small Japanese plants.

[SynapticBoomstick]

GODZILLA (1998) - The camera pans in, the baby bursts out of the egg, screams and... now walks the streets of New York with a cell phone and a latte. I don't know.

THE GOLDEN COMPASS and ERAGORN were definitely trying to start franchises.

I was glad Eragon fizzled out. You could use it as a textbook case in failed literary adaptation, so many lazy devices and horribly written.

[Ozzymandias]

Ozzymandias speaks: Doc Savage - Man of Bronze ended with an announcement of a sequel that never happened.

Ozzymandias has spoken!!!

[Couchtr26]

Spaceballs but more then likely just part of the joke.

A third Conan movie, it keeps leading on with that is another story. 

[ChaosTheory]

Spaceballs but more then likely just part of the joke.

We never got HISTORY OF THE WORLD: PART 2 either! 

[Couchtr26]

Spaceballs but more then likely just part of the joke.

We never got HISTORY OF THE WORLD: PART 2 either! 

True but I thought it was mentioned earlier.  Also, what happened to Manos 2: Electric Bugaloo. 

[FatFreddysCat]

We never got HISTORY OF THE WORLD: PART 2 either!  

"Jews In Space" totally deserved its own stand-alone film.
Jews in Space (Mel Brooks)

[claws]

The Monster Squad (1987)

They didn't outright promised a sequel but there was a strong hint/vibe for a sequel.

Kung Pow: Enter the Fist (2001) promised a sequel. They even added a preview of the sequel before the end credits. IMDb listed a sequel in production for nearly a decade before said info disappeared.

[Chainsawmidget]

The Monster Squad (1987)

They didn't outright promised a sequel but there was a strong hint/vibe for a sequel.

I've heard rumors that the original writer and director are planning to do a remake of it.

[Couchtr26]

The Monster Squad (1987)

They didn't outright promised a sequel but there was a strong hint/vibe for a sequel.

Kung Pow: Enter the Fist (2001) promised a sequel. They even added a preview of the sequel before the end credits. IMDb listed a sequel in production for nearly a decade before said info disappeared.

Yeah, what happened to that?  The life and times of Evil Betty must be known to complete the world. 

[HappyGilmore]

I actually wouldn't mind a sequel to Monster Squad.  A little late at this point, but still.

[JayJayM12]

At the end of Masters of the Universe, Skeletor pops up and says "I'll be back."  But he never was back.  He's just a liar.  A dirty, filthy, boneheaded (yep) liar.  I thought he was better than that...

Also, Bubba Ho-Tep promised a sequel, but, like a lot of others listed here, that might have just been a gag.

[BoyScoutKevin]

"Robin Hood" The 2010 version with Russell Crowe as the title character. The end of the first one obviously ended that way so they could do a sequel, which has yet to happen.

I don't know why there hasn't been a sequel. Lack of box office I would presume. I do know why the next one didn't get a sequel. Though, it was suppose to get a sequel. A script even being written for the sequel.

"Lair of the White Worm" was suppose to get a sequel called "Return of the White Worm," but when the film company making the original went out of business, that was the end of the idea for a sequel.

Anoither film that was suppose to get a sequel, but did not, apparently because of poor business was "I Am Number 4." And the book series upon which the first one was based, certainly sets up room for one or more sequels. One I'd like to see, as I really enjoyed the first one. Though, again apparently, I was one of the few who did.

And then there is the sequel, another sequel I'd like to see,  to "Hansel and Gretel: Witch Hunters." That one just may get made, as the end of the first one set up a possible sequel, and there is a listing for it on imdb.com under the title "Hansel and Gretel: Witch Hunters 2." And this one is a little more advanced than other possible sequels, as the idea is that the original witchhunters still living, plus a white witch and a witch hunter who is also a werewolf, would take on the great Russian witch Baba Yaga and her two sisters.