Originally Posted by cherokeesam
I think you've nailed it.
Conjecture, of course, so I won't spoiler-tag it, but maybe what happens is this:
*End of IM3: Tony test-flies his new Deep Space Suit. For whatever reason.....mysterious signal from space, possible new alien threat, he's going looking for Peter Quill for whatever reason --- anything. Turns out that Thanos is luring him into a trap, and captures him.
*End of GOTG: After all the events of GOTG, in which the gang presumably takes on Thanos unsuccessfully, they learn that he has an earthling captive: Tony Stark. Hell, just to mix it up even further: maybe he's even mind-controlled and enslaved to Thanos' will. Or maybe not --- that's just bonus stuff.
*Avengers 2: Tony Stark *is* out of commission. He spends, say, the first two reels as Thanos' captive, and is rarely if ever seen. That leaves the Avengers and the Guardians to team up and not only take down Thanos, but also the mission becomes much more "personal" (remember that quote....?), because they're trying to save their friend. Third reel features Tony back in action, and the whole gang gets to whale away on bad ol' Thanos.
This seems like a plausible route. I'm thinking that if the Infinity Gauntlet is involved though, Tony will be "killed" by Thanos at the beginning of A2 which starts out with Tony in space. Here's what I'm thinking:
-End of IM3: Sometime after he defeats the Mandarin, Tony is compelled to go into space after what he saw in the Avengers and the events that transpire on Earth in IM3. Basically, he wants time to get away from it all and remembers what both frightened and fascinated him in the Avengers, and decides to tackle it head first. Following a signal he discovers something related to the cosmic Marvel U (maybe another Chitauri ship, Thanos' rock, the Guardians, who knows)
-End of GotG: A scene involving Tony that picks up from where the IM3 scene ends. Tony is discovered by Thanos somehow who has possession of the Infinity Gauntlet now after the events of GotG.
-Beginning of A2: Tony is killed by Thanos using the Infinity Gauntlet in a sequence which involves the Guardians. SHIELD and the others will also be alerted by the fact Tony has been missing for a while.
In this scenario IM3's end credits scene chronologically takes place after GotG, though we won't realize it at the time. The Guardians will also be intimately involved in the plot of A2 right off the bat if they're present when Tony is "killed" and will have a connection to the Avengers. It also works as a direct sequel to the Avengers (as in you don't have to watch the previous phase 2 films) because it would be easy for the audience to accept that Tony went into space after witnessing it (Tony, not the audience) in the first Avengers. It would also pick up where the first Avengers left off with teasing the "death" of Tony only to actually do it in the sequel. Sounds very Joss as well.