They can't bring someone else in, the fandom will start riots and stuff, they don't want that kind of backlash. I think that he'll have to come in, as he's an original member and half the team is gone.
Each time Clint is there, they win so he's gotta be.
Right?
Plus Jeremy Renner was interviewed he said that he'll get lots of action, granted he made a joke out of it saying at he gets to bang all of the avengers but I think that may have been a cover up for what he really said, which is that he gets lots of action.
It's the biggest marvel crossover event in history, they can't not include him.