Light from above so Shatner wouldn't look like his HALLOWEEN Capt Kirk mask. Have a veteran producer ride heard on visual effects from day one. Lots more contrasting colors in uniforms and interior sets. Get somebody like John Frankenheimer to direct. There are about seventeen TRILLION other things I'd do to improve it, but to be honest, at least a few of them might mess up some of the things I absolutely adore about TMP, which still provokes the most schizophrenic responses in me.
Assuming we could go back in time and reshoot stuff, include the scene in Nogura's office, and put back Decker's line "We all create God in our own image," and probably make the uniforms a little closer to the original series style. And see about getting Shatner a better toupee, that Brillo pad he was wearing was distracting. And make the bridge stations wild so we can get better camera angles. If we're stuck with what we have, then put back the aforementioned line from Decker.
Improve it while keeping V'Ger? Well, a lot more colour wouldn't hurt the film. Better dialogue, change the red alert noise, cut out that rubbish with mimicking Ilia, not have McCoy beam in about 2 minutes after we've just seen a horrific transporter accident...a lot could be done. Just stop it being boring too. It's the only Trek film I'd not bother watching if it was on.
I'd probably have involved the other characters a bit more by sending in strike forces that didn't just consist of the leads. They could have had a bit of excitement fighting digitised alien guardians - possibly even the klingons - and learned a bit more about V'ger along the way. Obviously, sending in gun-toting troops for a fight would not have been in the spirit of the movie's contemplative 2001 vibe but it would have given the supporting cast more rope and could have amped up the action quotient. Kill a few redshirts, Spock still gets to meld with Ilia, maybe give Chekov, Sulu, and Rand a chance to do some shooting.
Three quickies. Fix the uniforms. Engage the original 7 in some constructive dialogue with each other. Shorten up the Enterprise travelling in V'Ger FX.
Time in Nogura's office, flashbacks with Ilya and Decker. We never see any sequence about HOW V'ger touches Spock's human side nor is there any build up in his conflict about it like the novelization does.
Trim 30 minutes out, take your pick of what 30 but I would suggest the trip through V'ger and the wormhole scene is about a minute or 2 too long, as is the re-education of Ilia. Add a touch more TOS music somewhere and you would have a better TMP, IMO.
They need to do some changes to the wormhole such as the sound, the interrior, and the color. Brighting or add more color such as more blues and greys to V'Ger's cloud. I wanted to see more distant shots of V'Ger's ship. Plus more complex information about V'Ger in the movie.
^ I would have Kirk in a fist fight every 5 minutes of the film. Change the warp drive engines to something absurdly large, and have the ship constructed on ground and not in San Fran but in Riverside. I would make the bridge very very bright with lighting and lens flares. Have the crew drinking beer in engineering. Lets have a villian that is bald with a bad attitube, just like shinzon with no motivation. Thats a good start.
Give Kirk a character arc. Make his conflict with Decker result in him realizing something about himself. Make Spock's suicidal spacewalk make him realize that he's just as pig-headedly been pursuing his own needs (the Enterprise) rather than the mission. In the end, make Kirk actually try to be the hero by sacrificing himself to join with V'ger, and be rejected in favor of Decker by Ilia/V'ger. Make more of the Decker/Ilia thing. You can't sympathize with him given how few crumbs we're given of their relationship. Give him dialog and action that makes us believe he'd give up his humanity to save the essence of the woman he loves. Make it so Kirk pulled in his old staff, so they're there because he needs back the Enterprise he left, rather than just having them conveniently stuck in the same jobs for years. Generally pick up the pace and the energy. Go for fewer visual effects but make each one matter more (spend more time on each). No Nogura! Adds nothing.
That's one of the best aspects of the pre-TMP trek movie Ellison pitched, that you have this mysterious figure going round kidnapping the original crew one by one till they are all together on the Enterprise and you realize Kirk is behind it. my fave part of Magnificent7-style flicks is 'gathering the team' stuff. FeatureTrek never really got that, though there is a tiny attempt in SFS and it was scripted for TUC.
I never understood people's obsession with that scene. It would have added nothing that isn't explained in the exchange with Scotty and Kirk in the shuttle. Plus, the shuttle conversation leaves the the Kirk/Nogura exchange to our imaginations. If it was as amazing that Kirk got Nogura's approval as implied, then seeing it would undoubtedly be disappointing and lackluster.