Downey is already being paid entirely too much money. But Downey is the draw of the movie. If he's not in it, I suspect the character will be put on hiatus. I don't think they'll go for a War Machine movie because no one cares about that anywhere near as much. Given that (a) Avengers 3 is still awhile away, (b) there aren't really going to be any Iron Man movies between now and then, (c) Downey is under contract for that movie, and (d) there are rumors of wanting to go with a "New Avengers" crew after that (e.g., Black Panther, Captain Marvel, etc.), I think they're preparing for the idea of going sans Iron Man after 2018 or so.