I don't see some of these movies happening, but here are some of my picks:
Iron Man 4
-Mandarin and the Ten Rings. All Hail the King hinted at a real Mandarin, and the Ten Rings are still out there. I'd like to see closure to this
Thor 4
-Enchantress would be my 1st choice. The Midgard Serpeant would be awesome, but that feels more like something someone is unleashing. I'd like to see Enchantress as the main villain.
Captain America 4
-Easiest one, bring back Evans for one more film if he'd do it, and send him off in the sunset with a battle against Red Skull!
Doctor Strange 2
-I would like to see Mordo and Nightmare in this one.
Spider-Man 2
-Mysterio!!! He's one of my favorites and I have been patient!
Black Panther 2
-Moses Magnum is an easy choice for this. If outside the box, Namor could also be interesting.
Black Window
-Spymaster is a good pick, but I also would like Madame Hydra/Viper
Fantastic Four
-I would not want Doom in the first film as the main villain. I could see a few working. Personally, Psycho-Man, Annihulus/Blastaar, or Mole Man all would work. I have a certain vision that would utilize the Frightful Four, myself.
Captain Marvel 2
-Supremor!!!!
Ant-Man 3
-MODOK would be well used here. Also would like Whirlwind or Count Nefaria.
X-Men
-Mr. Sinister. He can explain mutants to the audience and if they embrace his sci-fi, can showcase how different this new X-Men will be from the FOX version. I also would like Proteus.