I kind o have to vote for re-casting. It's the hard one, but also he most viable option. As comics go, team books are sold more for their lineup than their writing. You go in to follow characters you like, and a lineup shift can cost readers or viewers. Since the Avengers started with as close to the dream team as we could get, you aren't going to win a lot of people over if Cap Thor Iron Man and Hulk are replaced by Wonder Man, Black Knight, Sersi, and Justice. There's been some lineups of the Avengers and the Justice League that aren't remembered fondly, and hell if you want to see how well it could go over, look up the 90s cartoon Avengers: United they stand to see the Avengers without Cap, Thor or Iron Man. Passing the torch might help, but again, it's been done a fair bit in the comics always to the return of the original. Bucky didn't get a long tenure as Captain America, alternative takes on the hulk go back to big green and angry, Stark always puts the Iron underwear back on, and you really don't want to bring up the replacement Thor (Thunderstrike). They might work for a bit, but again, people come for the original character and characterization, and changing it seems like relabeling a bad idea under a good name.
Recasting can be rough, but the other options seems to bring some bad comic history to the movies.