I always find these exercises fun.
TMP - Tough to limit myself to just one thing I'd like to fix. Going to the area that needs the most help, I'd like the script to have an extensive rewrite before shooting. The main things I'd keep in mind for the new draft would be to improve the pace, give the characters more of the warmth & humor that they had in TOS, and add a little more activity & action for the crew (Spending most of the last 2/3rds of the movie on the bridge is, quite frankly, a drag.
Get them off the damn ship, already! Have V'ger threaten a planet, have them take out a shuttlecraft to investigate V'Ger, have them do
SOMETHING so that the crew is active rather than reactive.)
TWOK - Hmm. This is a toughie, as I can't think of much I'd like to change. Maybe keep in a line of dialogue about Saavik being half Vulcan/half Romulan, as that would preserve more of her uniqueness in subsequent films. She has more volatile emotions than Spock, but she's struggling to integrate them rather than repress them. That gives her a different note to play in the ensemble & gives us some setup for the future.
TSFS - I agree with
@Bry_Sinclair: Bring back Kirstie Alley as Saavik and pay her what she was worth. If that means paying her more than DeForest Kelley, so be it. As long as Alley returning as Saavik doesn't affect her joining
Cheers in 1987, I consider it a net positive.
TVH - Not my favorite of the movies, but I can't think of too much I'd change. I'd probably replace the music, as Leonard Roseman's score is my least favorite of the movie series.
TFF - Again, the problem is not the director (Shatner did a nice job and had some great looking shots), but with the story and the subpar effects. But if I'm limiting myself to just one change, I'd make it so that Sybok is not Spock's brother. He's never referred to again in any subsequent
Trek production AFAIK (I haven't finished S2 of DSC yet, so no spoilers, please) and he
really screws up Spock's backstory, IMO. I'd reconceive the character somewhat to make him a more fanatical Vulcan mystic who hears voices and may not be entirely sane. I think that'd give the movie a little more conflict and bite.
TUC - I'd like to put in Kirstie Alley as Saavik again. I liked Kim Cattrall as Valeris, but I think Saavik betraying Spock and the rest of the crew would be much more provocative and startling, and it would give definite closure to her character. (It would also let the "...A lie?" refrain throughout the film make much more sense, as it was a callback to a Saavik/Spock exchange in TWOK.) And since Alley would be returning to the role instead of a brand new actress being cast, she could have a previously-worn uniform in the proper colors. Win/win.
GEN - My gut feeling is "Do a true crossover film with everyone from both crews", but that probably isn't realistic. So, sticking with the basic story that they had, I'd ask for a rewrite where the Nexus was properly clarified. What can it do, and what can it not? Does it show you highlights from your life, like Kirk, or does it show you a life you never had, like Picard? What exactly did Soran see in there that made him spend 78 years trying to return to it? Does everyone leave an echo in there, or was that just Guinan? Was the Kirk that Picard encountered in the Nexus an echo, too? Can you leave the Nexus at any point in time, or are there limits? Hopefully answering these questions would also result in giving Kirk a more heroic and satisfying death.
FC - I'd probably have the two plotlines converge earlier, so that the movie wasn't just in the same A storyline/B storyline formula of the typical TNG episode. As it is, we've got a suspense/horror film aboard the
Enterprise-E and a more humorous romp on Earth with Zephram Cochrane, and they don't quite mesh.
INS - I don't think I've watched this movie since it was released, so it's tough for me to say what I'd change. Give it a rewrite so that it wasn't so damn bland and forgettable, I guess.
NEM - I'd probably change the director, since most of the problems on this one seemed to stem from Stuart Baird. This would hopefully result in Tom Hardy not being cast as Shinzon (I like Hardy as an actor, but he just doesn't look enough like Patrick Stewart for me to buy him as Picard's clone). If Frakes wanted the job again, give him the director's chair.