Quicksilver
This one is a tad bit tricky, because Quicksilver is dead in the Marvel Cinematic Universe (MCU) but alive and well in Fox’s X-Men universe. It’s a complicated matter involving rights and all that other boring stuff which no one cares about, but the fact is Pietro Maximoff was a goner after Joss Whedon’s Avengers: Age of Ultron.
It sucks, too, because Aaron Taylor-Johnson is a supremely talented actor and never got to show his full range in the MCU. While his sacrifice was undoubtedly the biggest of them all, his emo twin, Wanda (Scarlet Witch), got a free pass at survival – even if Elizabeth Olsen’s acting talents are better suited to Inhumans than a cinematic universe, but whatever.
With Evan Peters’ Quicksilver being extremely popular amongst fans, it’s unlikely that we’ll ever see Taylor-Johnson in the role again. And it’s a real shame, too, because if the MCU never had major plans for Quicksilver to begin with, they should’ve cast someone else and saved Taylor-Johnson for a significant part in another movie.