This should be fun:
X-Men
-Establish codenames as a trend among mutants. Some mutants gave themselves wacky codenames after their powers came up, and it caught on. Xavier and Magneto took this trend and ran with it. Marie gave herself the nickname Rogue because she was a runaway and she didnt want to give away her powers.
-Wolverine and Sabretooth know each other. It could be as simple as Logan telling Charles, weve met. I didnt know hed gone mercenary. Also, they exchange banter in the snowy forest fight before Sabretooth knocks him out.
-Stop sending X-Men out in pairs. If anybody goes out on assignment, it should be three or more characters on the job.
-Dont kill Gyrich or Senator Kelly. Theyre too important to just throw away as cannon fodder.
-Make some mention of other X-Men who are on assignment.
Iron Man (Since this is the best one weve gotten, I only have a few minor points).
-After his return from captivity, Tony gets some kind of charity thing going in Yinsens name.
-It seemed kind of dumb for Tony to suggest reviving the arc reactor thing to Stane, then hoard it for the Iron Man gear and give Stane nothing whatsoever to show to the board of directors. Maybe design some specs that only produce half as much power as the armor, just so hell still have an advantage if someone tries to misuse it.
-Give Obadiah Stane some kind of scheme that hes running beyond building his own tech-suit, something that Iron Man must stop.
-Get an action director to help Favreau out.
Spider-Man
-Have Peter stay in high school for the first three movies, and have him graduate at the end of SM3.
-Introduce Gwen in Part 1 as well as MJ.
-Have Spider-Man crack jokes while in costume.
-Let the burglar live in Part 1 (and while youre at it, dont have Peter rip off his damn mask.)
-Like Iron Monger above, give Green Goblin some kind of scheme that Spidey has to foil. Worked in SM2.
X2 (Just two for this one, since its already the best one.)
-Wolverine cuts the sleeves off his uniform. Just because.
-Theres a wonderful pair of deleted scenes from the DVD where Jason, probing Xavier, makes Xavier THINK that he and Cyclops escaped, and they head back to the mansion where Xavier finds the little girl with the multi-colored eyes whos really Jason. Put those back in, cause thats good stuff.
Daredevil (Im starting with the Directors Cut so I dont have to cheat.)
-Make Karen Page the primary love interest and save Elektra for the sequel.
-When you do introduce Elektra, shes an assassin, not a girlfriend.
-Like Q said, have Stick train Matt as a kid.
-Kingpin does NOT find out Matts identity at the end.
-Cast somebody who actually looks Greek for Elektra.
X-Men Origins: Wolverine
-Switch out Deadpool and Maverick/Agent Zero, by having Wade in the chopper that Logan destroys. He survives but gets horribly burned up, setting the stage for Deadpool.
-As Wolverine and Gambit fight, they are attacked by Sabretooth and Maverick, forcing them to team up.
-The Weapon-XI that Wolverine fights is Calvin Rankin, aka Mimic, not Deadpool.
-Logans memory is erased through more technical means than a bullet to the head. And maybe, just maybe, he fights Weapon XI AFTER the memory wipe.
-Replace John Wraith and Bolt with a couple of other better-known mutants that I havent decided on yet.
X3: The Last Stand
-Split it into two movies. Cure saga first, then Dark Phoenix second. Part three ends with a cliffhanger where Jean returns. (As such, I am cheating and turning my remaining 4 bullets into 8, because thats what this movie deserves).
-Wolverine fights Marrow in the forest instead of Spike.
-Nightcrawler is a main character, sitting in on conferences with Logan, Beast, and Storm.
-Make Jubilee a character, with a part in the Danger Room training and hanging out with Bobby, Peter, Kitty, and Rogue throughout the movie.
-Bring many more X-Men along for the final showdown, including Nightcrawler, Jubilee, and the still-living Cyclops. Rogue arrives soon after, along with Angel, maybe Gambit, and a whole mess of mutants from the cure clinic to back up the X-Men and even the odds.
-Pyro wears a jacket with flames on it. Come on, people, SOME attempt at a costume.
-Colossus battles Juggernaut as Kitty works to find Leech.
-On the subject of Dark Phoenix, if she uses her power on Scott at all, it merely knocks him out for most of the movie. If she still eviscerates Xavier, then Charles will merely leap into Jeans mind and hide out there for a while until the opportune time.
-Give Angel more screen time.
Hulk/The Incredible Hulk
-Just one: Transform Ang Lees Hulk into the origin movie that precedes Incredible Hulk.