Wolverine

It goes without saying why Hugh Jackman's Wolverine cannot be replaced in the X-Men shared universe "“ at most, his role could be substituted with the one of X-23, even if that would make the X-Men timeline even more nonsensical.

However, the Marvel Cinematic Universe hasn't introduced a superhero like Wolverine until now. Of course there are the fighters (such as Captain America) and the angry ones (go figure). But Logan is one of a kind, with his stamina, his healing factor, his tracking skills, and so on. Furthermore, with Disney, the character's true potential from the comics could be explored "“ after all, Wolverine is regarded as one of the better hand-to-hand combatants of the Marvel comics.

So if you think that the fight between Cap and Bucky in Captain America: The Winter Soldier was an intense one, imagine having Steve facing a literally indestructible foe (yeah, he'd have to be one, at least until he regains his memories).