Predator

Short version: After the Aliens aliens, who's next?

Aliens vs Predator was one of the first such crossover movies. But that premise can keep going... within the Predator-verse itself.

There are plenty of other beasties on Earth and elsewhere aside from xenomorphs, from other movies and series, that the Predators could hunt. After all, the premise is that they are hunters always looking for the next species to add to their, um, trophies. 

Like Pokemon trainers, except they gotta kill 'em all. 

The first potential Predator prey that came to my mind were the worms from the Tremors series. While the Predators are unseen when they can become invisible, the worms are unseen simply because they are underground. And they don't need to see to hunt-- they use the vibrations from footsteps to know where to pop up. Invisibility will not help the Predators here, and they will have to change tactics.

Similarly, the sightless beasties from A Quiet Place hunt by sound, so again being unseeable is not going to help.

Other Predator prey to consider: the beasties from Critters, or from Species, the CHUDs, the Tommyknockers, the aliens from Independence Day...

And if you really wanted to have fun, have the Predators take on the Gremlins! Those little buggers are pretty crafty when they want to be. It might be a hoot to have the Predators pick on someone less than their own size. 

Of course, the Predators are aliens who mean no good for humans. So we could also send the Men in Black after them. At least the MIB weaponry and tech would offer a fair fight.

I can see the Predators hunting their way across the history of beastie and alien movies. 

Harry Potter

Short Answer: Back to Hogwarts

The Fantastic Beasts prequel series is... not going well. Perhaps it's best to close the (text)book on all that and go back to school.

We see in the epilogue of Hallows 2, we see our heroes' kids go off to Hogwarts. That movie came out in 2011.

So, we have two options. One is to pretend those years did not pass in the story, and we see the adventures of Harry, Ron and Hermione's kids at the school, alongside the descendants of their rivals and other classmates. The other is to acknowledge the passage of time. 

Either way, we see that after Dumbledore passed, McGonagall was made headmaster (headmistress?) but now she has retired and Hermione has taken over.

When whatever evil we can reasonably say is left unresolved even after the first 8 movies comes back to haunt the school, she enlists Harry's help. Harry is mostly done with the magic business and really is OK with never seeing the school again, but for her sake, he agrees. Ron, being married to Hermione, is already there. 

Their children are either-- depending on the passage-of-time situation-- students or faculty at the school. As parents, the original threesome are naturally protective of their children and at first hide news of the danger. When it becomes clear their cover-up efforts are failing, they still tell their kids to stand down and let them handle it.

This is not what happens. The kids do help, but end up needing rescuing. Still, their time with the enemy reveals its weakness. Now they are feeling protective of their parents and try again, but their parents are also on the attack.

Their schemes run afoul of each other, but then the two generations team up. The parents learn that their children are brave, capable, and clever. The children learn that their parent's experience and wisdom are also valuable. The enemy, faced with this double-barreled resistance, withdraws.

Victory! For now....  

Pitch Perfect

Short Answer: Glee crossover

The inherent problem with any franchise set in a high school or college is that, even if your protagonist is a freshman, you can get at most 4 movies out of the series, one for each year. You had better plan for that by the sequel, bringing new freshmen into the storyline in Part 2, so that they can be there for Part 5, because your hero needs to graduate in Part 4.

There are 3 Pitch Perfect movies so far, and they can't go beyond 4 because the older Bellas will have graduated by now, and those who were freshies in Part 1 will graduate at the end of Part 4.

The other solution? See what their lives are like post-graduation. Surely some of them will go on to be glee-club and choir directors and at places like...

McKinley High, home of New Directions. Also nearby are their rivals, the preppie Warblers. 

So it stands to reason that some of the Pitch Perfect alums (or those of their rivals, the Treblemakers) may start teaching the New Directioners (or their rivals, the Warblers).

While, sadly, several of the Glee cast members have passed on in real life, the remaining actors still available.

A cast consisting of former Pitch Perfect and Glee singers, plus new cast members, will extend both series... and make new stars.

Imagine McKinley High with Sue Sylvester as the superintendent, Will Schuester as Principal, and Aubrey as the head of New Directions. Bumper, having flamed out in pop music, now runs the Warblers. The story also encompasses what's happening at Barden, where some former Glee cast members like Mercedes and Artie now teach music. 

Karate Kid

Short Answer: Miyagi's story

Over the course of a trilogy, a reboot, a remake, and a TV show, we learn about Mr. Miyagi's life story in dribs and drabs... but wouldn't it be something to have one or two movies in which we see all of that happen? 

In order, with all the missing pieces filled in? And all of the experiences that led to the Miyagi we have loved for nearly 40 years? 

Think about all we would learn along the way about a too-neglected part of American history: Asian-American history. 

We know so much about Miyagi's life that the screenplay is almost pre-written. And actors would line up to play "Young Mr. Miyagi."