I wouldn't even think that would be an issue with Iron Man. Short of another alien invasion, Tony's ego would prevent him from asking for anybody else's help anyway. To me the only issue future movies have to deal with is why Fury and SHIELD (who seem to be aware of everything that's going on) don't intervene and force Thor, Cap, etc to help out during one of those huge, city-wide battles that happen in nearly every superhero movie. Thousands of lives are being endangered by some rampaging monster or supervillain, and Fury is just gonna let Iron Man or Cap handle it on their own? Really??