RDJ is pushing 50 right now, and Mark Ruffalo issn't far behind him. So hypothetically, if they were to start relatively soon, like in the next couple years, Chiwetel would still be a great choice.

As far as the big name thing, it isn't really necessary.

Hemsworth was practically an unknown. Chris Evans may have been familiar to audiences but was by no means a star. He wasn't carrying any blockbusters as the lead. Even RDJ wasn't a "superstar" until IM hit. Shit, go outside Avengers and it's pretty much the same. Hugh Jackman was a nobody. Who was the biggest name in X-Men when it was released? Halle Berry? The bald dude from Star Trek? Tobey Maguire's biggest hit before Spidey was what,....oh wait, he didn't have one. Until the American Psycho superstar Christian Bale put on the cape and cowl the biggest name cast in the lead role of a successful superhero flick was Wesley Snipes(Daredevil may have turned a profit, but it's not a success).

Superhero movies typically have not needed a huge name to open well at the box office. Granted, being that BP is a black superhero a bigger name would help, but considering the success of anything Avengers-related, it would pretty much be a guaranteed hit.