DC will have to manage to come up with more than two successful comic book movie IPs before they can try for Justice League (presuming that Man of Steel is successful, of course...) They could do a Batman/Superman movie any time, though.

Iron Man 3 would probably have Tony fighting something Thor can't fight, like...a wizard.

An alien wizard! With technology disguised as magic installed in some kind of apperatus, like wands or rings. And he will use this technology and intellect in an elongated gambit for the world. Because Thor has never been good with mastermind villains, hence why Loki is his arch.

The Mandarin could probably give him a run for his money, after all.

They alluded to the Mandarin in the first movie so I figured they had to bring him up one day.

I read this interview they did with the guy they've lined up to do Iron Man 3 and he said he wanted to make it like a Tom Clancy thriller which could work very well. There'd be no reason to involve the other Avengers in a conspiracy but I could see SHIELD being a part of it. Plus, post-Avengers productions might realize they have to scale back so we don't keep asking, "Why don't you just call Thor?" and this could lead to more personalized stories.

But that's all here-say so we got no clue what we're gonna be in for.

Only when he's Tony Stark
 

Wouldn't it be ridiculously easy to make it so that the Avengers kind of hate each other? I mean Tony's a dick, Thor is never available, and what do you say to a man out of time?

Especially when said man out of time knew the dick's father (just a guess).

Possible. The Avengers were united in their pursuits, but often separated by their ignorance. Particularly The Hulk and Iron Man. One cannot count using only his appendages the amount of times Iron Man thought the Hulk was hulking out and just started shooting him to try to stop him.

Civil War plotline for Avengers 2?

I would call that a terrible idea but I could see it being a logical continuation. The trouble is that they'd have to shoe-in an instigator and it would probably come from on of the character's separate movies. Like Thor having a battle that levels an entire city or something like that. And they really haven't established enough heroes yet in order to make it work.