The characters of Hawkeye and Black Widow wouldn't need to be recast. Just drop them from the scrip if it comes down to that. With the rumors of Quicksilver and Scarlet Witch being in the film, they will have enough characters in the smaller roles. Might be better off this way too as I wouldn't want the cast of characters to get too large. Now the rest? Marvel better give in for the larger parts. No way should they recast the Hulk again.