It just seems to me like the point of an ensemble movie is that you can incorporate some of the more minor characters who couldn't carry an entire movie by themselves.
Thing is, though, that this isn't a start-from-scratch ensemble; they've got three guys who will all have had one movie (in Tony's case, two) to themselves. The audiences will be turning out to see them.
I'd include Black Widow for sure, but otherwise I wouldn't push it further for a first movie.