We call this “the X-Men pitfall”. The first X-film had a core group of six good guys and four bad guys (two of whom barely did more than growl). By the third film we were up to ten core good guys and six bad guys. X-Men: Origins – Wolverine, about that famous lone wolf Canuck, had 10 core characters, about eight more than we wanted to see. X-Men: First Class had 14 principals. The Avengers is already running with six team members and two (remaining) support staff, not to mention various love-interest hangers on from the individual franchises and whatever villains rock up to the party: we do not need more than that. The great strength of the first film was that it focused on character and the interplay between the team. Surely there’s more to play with there without adding new Avengers to the mix? But if you absolutely must add someone, make it a girl.