Back To The Future 4: In this sequel Marty will have become the Doctor to cure his Parkinsons disease, of Biology but he will obviously work on reiniating time travel research, what he doesn’t know is his research is being watched and hacked by Biff in his timeline, Biff will have become president in the dark timeline that was introduced in the second movie (it will be revealed that there are multiple timelines created due to timetravel, something that Doc Brown didn’t anticipate).

Basically Biff will commit the US’s military industrial complex to figuring out timetravel and will conduct transdimensional espionage, recruiting his alternate self from across the multiverse to steal the secrets of time travel from now Dr Marty and send commandos through time to hijack the train that Doc Brown built, last seen in the third movie.

If I can get it made then all the original actors would reprise their roles and the part of Doc Brown’s sons would be played by Christopher Lloyds onscreen father in the movie Nobody - Bob Odenkirk, with his brother hopeully being his onscreen brother Michael McKean (from Better Call Saul)

Truman Show 2.0

I can’t believe this one hasn’t been made… after Truman escapes he has a child with the love of his life, only for the business execs from the first movie to pay money to secretly make Trumans son the next Truman, Truman 2.0 if you will. They use advanced AI tools to obfuscate Truman 2.0s real identity and location from the general public and to hide what they are doing from Truman himself - he hasn’t escaped but the true focus is his son not the trueman.

I have several more.. mostly rewrites of bad movie sequels