Re: Avengers Sequel ideas?
Well, we know the first movie will be Loki, plus whatever else he creates as a distraction, as it was in the comics. An ensemble villain (like the Masters of Evil) require too much screen time to make believable, so they're out. Ultron's a great choice, but I personally would prefer he be a villain for a Giant Man film, since Ultron's story is so fixated on Hank, and making Hank, a new character, the center of a sequel is a bad idea. The thread of the story should hang on the central characters: Fury, Cap, Thor, Iron Man.
An alien invasion is a pretty cool way to do things. Having the Kree and tying in Ms. Marvel could be pretty awesome, and their alien technology can be used to involve other heroes as needed. You still can't have a cast of ten Avengers and make them all feel like real people as opposed to cartoons, but you can add one or two more as part of the storyline.
A third one, if I can imagine such a thing, would probably involve Kang, and be a great way to cap off or bring together all of the Marvel films, period. Good times.