They'll probably try and do it that way. All they really need to do is make the ending open ended enough for each film that they can be part of the Avengers. Like Thor, the cube didn't really have any part in the film until the end when the scientist opened it up and was controlled by Loki. The other films just had RDJ or SamJack appear in the end.
I do think if the other franchise can't keep up or make as much (since the actor will likely ask for a raise) then they might just not make the films and just add them to the Avengers, kind of like the Hulk film. They didn't really bother making another one when they had plenty of time to put one out before the Avengers film.