It's actually a lot harder than it seems to correctly assess which actors would be good at which parts. A lot of Marvel fan-casting efforts boil down to picking performers who look like the character or recalling similar presences in other films. There are beloved performers who have left fans disappointed. Other stars have appeared against type and immediately redefined their character. Robert Downey Jr. left his role as the most popular character in the MCU, only to return in a near-opposite role. However, it's still interesting to speculate how his other roles might impact that change.