Within both Marvel and DC comic book continuity, superhero mantles are constantly being passed down to those equally deserving, once the originals decide to hang up their shield, or gauntlets, or hammer. So why not adopt the same trend in the Marvel Cinematic Universe? Marvel have already released one mantle-passing movie in Ant-Man, and there is now precedent to start replacing actors who don't want to continue.
In the comics, when Steve Rogers dies, Bucky Barnes takes up the mantle - at least once he shakes off his Winter Soldier mind-control conditioning. Following that, and more recently, one-time sidekick and fully-fledged Avenger The Falcon has adopted the shield, blended with his unique fighting style and aesthetic. Either of these are a natural option to replace Chris Evans and Marvel would also reap heavy rewards in the media for having an African-American Captain America on the big screen.
Likewise, in the comics, Rhodey takes over the Iron Man mantle once Tony Stark is unfit to continue on. And in Don Cheadle, Marvel have an Oscar-nominated actor who could more than capably carry the weight of a franchise heavy-hitter on his shoulders. He's also already proved he has the screen presence and personality to carry it.
As far as Thor goes, Chris Hemsworth shows little sign of growing disinterested with the franchise; yet, in the eventuality that he did want to depart the now-hallowed halls of Marvel Studios, we have a new, female Thor inhabiting his role in the Marvel comics continuity. And she's already in the MCU as well, of course.
Cinephile since 1993, aged 4, when he saw his very first film in the cinema - Jurassic Park - which is also evidence of damn fine parenting. World champion at Six Degrees of Separation. Lender of DVDs to cheap mates. Connoisseur of Marvel Comics and its Cinematic Universe.