I disagree that Hawkeye had a minor role in Avengers, dude was practically Lokis right hand mana nd single handedly took down the helicarrier. He'll have a decent or the same amount of screen time if not more in the next one.
I don't see Nick Fury having a minor role either especially if Ultron is a SHIELD creation, unless they kill him of in Winter Soldier but even then apparently in this world (just like the comics) no one will stay dead. Hell I actually forgot that Coulson might return to A2 as well.
As for villains, I don't mind that Baron is there for 10-15 minutes but in a 2 hour film, that is long screen time. But the focus is on Ultron so at the end it doesn't matter but that is a lot on their table.
At the end of the day, minor role or not, it's still another hero to fit in and it doesn't matter if they take up 4-5 minutes of screen time, which I doubt (outside of War Machine). Superman/Batman only has 3 confirmed and 2-3 more rumored to have a cameo and it is already being torn apart.
Phase 3 they may be on some Avengers Disassembled **** leading in to A3 anyway. I expect solo movies from those other guys and they don't have to be in Avengers.
Solo movie or not, they will still likely be all in for Avengers 3 especially if it all leads to Thanos. I mean it makes sense to take out a big villain as Thanos but it's hard to pull that off with so many characters and sharing all those screen time. Whedon did a great job in Avengers pulling these guys together but there is a point where too much is simply too much. Also seems like a waste to have solo films for Strange and BP and not be in Avengers 3, same with GotG since they are pretty much a set-up for Thanos. We know for sure Iron Man is there. The only two dispensable characters are Scarlet Witch and Quicksilver since they'll likely won't have a solo film at all and they can come be forgotten for a few years and be back for Avengers 4 to deal with Scarlet Witches power or something.
GOTG aint gonna be in Avengers 2 at all. At most a post credit scene. Adding some space **** to an Ultron movie is doing too much.
That's why I said "could still be", though not likely but still a possibility.