I'm not going to say they should've made this or that movie better since while I certainly agree with that for several movies, it's kind of a pointless criticism.
What I would've done differently specifically is:
1. Make the original Ant-man the first 'Ant-man and the Wasp' film. That movie would've been so much better with both Scott and hope suiting up and it was kind of a travesty that Hope didn't even get to be a hero until after Infinity War, especially since the MCU was so desperately in need of more female heroes anyway.
2. Release Captain Marvel much earlier and give her a real role in Infinity War instead of the bs fake role in Endgame. Or don't release it until after Endgame. Either could work.
3. Put a bigger leash on Waititi. Ragnarok was ok. Love and Thunder was better. Both suffered from Taika's lack of filter.
4. More money/better work processes for FX, especially for Black Panther and She-Hulk (which were not the worst fx in the mcu, but definitely suffered the most from bad fx).
5. Scrap Iron Man 2 and 3. Or completely rework them, at least, but Tony was so much better as a supporting/ensemble character anyway.
6. More flexibility on D+. Hawkeye and Moon Knight should've been movies. Ms. Marvel should've been a longer, less serialized show (kind of like She-hulk). Falcon and the Winter Soldier had the right format, but needed more development (and in fairness, was hurt by rewrites forced by the pandemic). Also, I want more D+ specials. Something, fun, off-the-wall, short and sweet and not too expensive. And release the D+ content on physical media already, dammit.
7. A team-up movie before Quantumania. I was theoretically on-board with Kang Dynasty being the next big team-up when it was newly announced even though it was clearly a risk, but the risk has definitely not paid off with all these delays (and not all of them are out of Marvel's control). And there have been a LOT of new faces added to the MCU who are just kind of dangling out there waiting for their next appearance.
8. Put Hulk's smart transformation back into Infinity War. Or a flashback in Endgame. Or release a special or something. Don't leave his entire character arc off-screen.
9. Get a handle on the post-credits scenes. I adored phase iv and, so far, v as well, but the post-credits scenes have been terrible. And too many of the shows don't even have one (or have one tacked onto a random episode in the middle where you can't find it and then it's just a generic tv cliffhanger, anyway).
10. Put the breaks on Spider-man. His movies don't have to come out every two years. Give me a new Spider-man every three or four years so I can also get a new Dr. Strange or Shang-Chi in less than 5 years.
11. And finally, this is the one I might still be persuaded to change my mind on and it's definitely going to be the most controversial one but: Nick Fury should've died in Civil War. Civil War is great, but didn't go far enough in driving home the damage. It needed a good, tragic death - of someone beloved by all the Avengers and the audience, too. Fury not only fit the bill, he even made the most sense narratively, as someone who would naturally be in between the two sides, anyway. And, let's be honest, his continued existence in the mcu isn't exactly amazing, either. He was great in Avengers, Winter Soldier and Captain Marvel. The rest of his roles have been just treading water as the marvel guy who just kind of has to be around because its expected. I'd much rather he had a great dramatic end that helped to make a good movie truly great rather than degenerate into the bore fest the character's become.
