5. Nick Fury
Hollywood mega-star Samuel L. Jackson surprised a lot of people when he agreed to appear in Agents of S.H.I.E.L.D. in cameo form as Nick Fury. It was a brilliant reminder of just how good the series could be - and for that reason it would be great if he did so again. That's a real possibility, as Jackson has explicitly stated that his part in Avengers: Age of Ultron is minimal - thanks largely to the fact that his character is merely human and the movie is very much about super-powered heroes battling super-powered villains. The Agents of S.H.I.E.L.D. - for the most part at least - aren't super-powered, but will undoubtedly be playing their part in the fight against Ultron and Nick Fury could definitely help them with that. Whether he provides them with equipment, shelter or merely advice, his character - who has been in several movies in the Marvel Cinematic Universe to date - would provide an almost palpable link to the events of Age of Ultron.