They don't need big name actors. They need GOOD actors.
It's easy for people to forget that Robert Downey Jr... while known to the public for his boozing antics of youth... was NOT a big name actor. He'd been in very few movies, and none of them in recent years had been a big success or very popular. But when he was cast, it was perfect because it was easy to tell, "This guy can pull off Tony Stark."
Now Ed Norton is a big name; that's for sure. But not every hero needs to be a big name. They need to be actors who FIT the role. Norton and Downey both did that.
If you want a hero movie with a big name actor that failed, look no farther than Daredevil or Ghost Rider.
So while having a big name actor is a nice bonus, you need someone who can actually pull off the role convincingly. If it takes a lesser known actor to do that, then that is what must be done. Take a look at Wolverine. Hugh Jackman was not known in America AT ALL prior to the first X-Men movie. Now he's a huge star, and most people think he's the perfect Wolverine.
Just have to find the perfect actor, not the biggest actor.