He didn't even say it was a team-up movie at all; that's just an assumption. Maybe they've thought of a new way they can "come together." Like, maybe an arc where the different heroes all separately fight the same villain, with each film's events influencing the next. That could be a cool way to introduce Dr. Doom, say -- maybe he's a power behind the scenes in the first couple of films, then he emerges as a more overt adversary in the next few films after he's exposed, and it all builds up to a final confrontation with the Fantastic Four, who have perhaps been seeded over the previous few solo films.
