3. Laurence Fishburne (Nick Fury)
This would be a nightmare to recast if only because the image of Samuel L. Jackson is so deeply ingrained in the mythology of Nick Fury himself; his comic book likeness was altered in the early 2000s to resemble Jackson, which made it a slam-dunk that, were a series of Avengers movies ever to be made, he would naturally end up playing the character. Even putting that to one side, Jackson is an extremely popular actor and in a sense the lynch-pin holding The Avengers together; there's no guaranteeing that another actor would have anywhere near the same chemistry with this cast (or a new cast) of actors. But the next best thing would be Laurence Fishburne; aside from the obvious point that he's the same race as Jackson, Fishburne is a fantastic actor and would certainly bring some impressive chops to the table. As Morpheus in the Matrix trilogy, he demonstrated a tremendous knack for ripping through dramatic dialogue, and would doubtless do the same here. The only question mark is how well he'd fare in terms of action, but if Jackson, who is over a decade his senior, can do it, then we shouldn't need to worry.