If you’re looking for the ultimate character to tease major events in the MCU, Bishop is the man.
For example, Bishop once tried to kill Kitty Pryde and Cable once, even though all three are generally seen as heroes.
Bringing a character in like this could add a level of tension to the MCU I could really get behind.
Traditionally the character travels back in time to meet with the X-Men and potentially alter the events of the future, which are almost always post-apocalyptic.