Captain America 2
The opportunity for Captain America 2 is unique in the sense that it is a sequel that can act like it isn't one without having to do an origin reintroduction because the setting will be completely different. Captain America: The First Avenger, of course, was set in the 40's in WWII. The sequel will be in modern times, probably around the time the Avengers are going to be formed. That pretty much means they've got free reign on who they want to use, because probably apart from flashbacks, no one from the first movie's going to be around.
The Falcon - Sam Wilson isn't exactly the most famous Captain America ally, but given his power set, it would be cool to see how Cap would interact with him. He could play a very minimal support role, as Cap's recon. It'll be interesting to see if Falcon is introduced as the guy to bring Cap back into the world, not only as a superhero but as a regular guy (who can fly and talk to birds). Falcon will be a friend, more than a sidekick, like he is in the comics.
Thought maybe Nate Parker could play him. He's got the build and the height. The acting chops, I guess he could pull it off. Another fantastic choice would be Michael Jai White. You know - Spawn? Jax? Yeah, that guy.
Sharon Carter - We got treated to a very spunky Peggy Carter in Captain America: First Avenger. Seems only right to have her grand daughter in the mix as Cap's love interest. No, that's not creepy at all!
Just like in the comics, Sharon's admiration for Cap will blossom into steroid-induced love. Of course, she'd be a SHIELD agent, just as she was introduced in the first movie. I think Amanda Righetti should play her, still. I mean, there's really no reason to change it up, is there?
Bucky Barnes - This one's tricky. Do we really want to see him return in Cap 2 already? In what capacity? Now, I thought about it. I do want Bucky in Cap 2, but not as his sidekick. I don't even want him as the villain, as the Winter Soldier. At least, not yet. Bucky should show up after the credits, as a master assassin working under Department X. It sets up Cap 3 - Captain America VS Winter Soldier! Of course, no one else can play him except for Sebastian Stan.
Crossbones and Baron Zemo - Here we go, villain time. Now, I know one of the biggest pitfalls of the sequel syndrome is introducing two or more major villains in one movie. It happened in Iron Man 2, The Dark Knight, Batman Returns, Spider-Man 3 and the list goes on. What happens is that the villains become generic baddies because there are too many of them and they can't be given the proper screen time for character development. If that's so, why am I suggesting we go with two villains? Well, Crossbones is the muscle to Zemo's brains, until he faces off with cap and proves that he's got muscle of his own.
Crossbones is a lot like Cap - he believes in something and is willing to fight for it and he's got skills to back it up. Zemo, on the other hand, is a mastermind genius who can hold his own against Cap. It'll play out like a weird partnership riddled with mistrust and deception.
I cannot, for the life of me, think of anyone else playing Zemo besides Christopher Waltz. For Crossbones, it's a little harder to cast. It's supposed to be some big dude in a skull mask. Could be anyone, really. So, yeah. Jury's out on this one.
Thor 2
Thor had a smorgasbord of supporting characters - everyone from Sif to the Warriors Three. Even Donald Blake was in the movie (Well, kinda). There really isn't anyone new I'd like to see, except for maybe a new villain.
However, I would like to see a little more from the Warriors Three. The deleted scenes were mostly of them, which had a lot more characterization. They're more than just Thor's sidekicks; they're three of the most loyal warriors to Thor.
Balder - The relationship between Thor and Loki as brothers creates so much drama and their interaction exudes tension between them and towards other people. Loki may be a little evil, but he's still Thor's baby bro, and Thor looks after his own.
Brotherhood is something a lot of people can easily empathize with. Know what might make that whole conflict better? Throw a monkey wrench in there with another of Thor's half-brothers, Balder. For casting, I haven't the slightest clue. Anyone got any idea?
Tyr - Norse god of war! Just make sure he has only one hand. Tyr is older than Thor, so he could be a trainer figure to Thor. Even though he isn't all that necessary, it'd be cool to see him in there. C'mon, at least a cameo! Casting? For some reason, I can't help but imagine Mickey Rourke as Tyr, but of course, that's impossible.
Hela - Let's make it a family affair! Let's throw in his niece in there, too! Loki's daughter, Hela, is the guardian and keeper of Hel, the Norse equivalent of hell. Since Loki's your main bad guy for Thor 2, he should have an ace up his sleeve. In Thor, it was the Destroyer armor, which went down way too quick. For Thor 2, it should be Hela. Act 3 could be entirely in Hel, confronting dead Asgardians, yeah? Ooh yeah.
According to IMDb fantasy casting, the best one to cast would be Rachel Weisz. AGREED!
Iron Man 3
We already got War Machine. So, there's that. We're done here. More War Machine please.
I guess apart from Rhodey, Pepper and Happy, Tony Stark doesn't really have a lot of supporting characters. I guess it's because The Avengers are his supporting characters. I kid, I kid.
Rumiko Fujikawa - Stark is the all around ladies' man, and rarely does he meet a girl that can take him for a ride. Rumiko was a strong willed girl who was just as ambitious and wealthy as Stark, unlike most of the bimbos he takes home. It'd be cool to see how Pepper reacts to Rumiko's flirtation with Stark, to make for scenes that are thick with bitchy passive aggressive behavior.
I know Rumiko's Japanese and Zhang Ziyi is Chinese, but I don't know anyone else who could play her. Enlighten me?
Blizzard, Melter, Whirlwind and Living Laser - Okay, I may be going overboard here naming practically all his classic B-list enemies, but I don't expect them to play big roles. They'll be Mandarin's men, mere cannon fodder for Iron Man. Super powered goons being taken down with repulsors and unibeams would be nice to see.
Spymaster - Or, if we wanna go with a more competent antagonist with a totally different function altogether, I say we get Spymaster as the guy working for Mandarin. With Tony Stark's industrialist background, a corporate spy and saboteur would be a perfect opponent for him. They could take some inspiration from the comics, too, and have him kill Happy Hogan.
Avengers 2
There is no question that the biggest missing characters in The Avengers were Ant-Man and Wasp. They were founding members! We do understand that there really was no time to introduce Ant-Man and Wasp before the big summer blockbuster so there was no way they could've been included. Still, a Dr. Pym cameo would have been very much appreciated.
I hope against all hope that Edgar Wright wasn't just collectively trolling us.
If they do go down the Hank Pym/ Janet Van Dyne route, these characters have to be as intrinsically flawed as they are in the comics for them to be interesting and to set them apart from the other more popular costumed heroes they run around with. Pym's got a lot of self esteem issues and Janet, in her early years, can be a bit of a prima donna. Also, marital problems.
For casting, I can somehow see Nathan Fillion play a good Hank Pym. Either that or I just really wanna see him in a proper comic book movie. For Wasp, Kristen Bell could be a good choice. Who would you cast as Wasp?