With the Star Wars universe expanding in all directions, and the fact that the end of The Mandalorian Season 2 actually gave us a look at a post-Return of the Jedi, pre-The Force Awakens Luke Skywalker, many have wondered if we might see more of Luke Skywalker in future films or series. Mark Hamill has said he's done with the role, but if there really was a plan to tell a bigger story with Luke, CGI likely wouldn't be the appropriate way to do it. Some new actor would need to be brought in, and many fans think the Winter Soldier himself, Sebastian Stan, would be an excellent stand-in for Mark Hamill.
The joke that Sebastian Stan looks remarkably like Mark Hamill, and thus could easily play Luke Skywalker, is one that both men have had fun with in the past, and Stan has said that he would love to be part of Star Wars if that opportunity ever came his way. However, the Marvel actor recently said that actually playing Luke would require something from Mark Hamill, and Hamill himself points out that while he might play Luke in movies, he doesn't really have any actual power.
To be clear, Sebastian Stan didn't actually say that Mark Hamill needed to be "involved" for him to take the Luke Skywalker role. Rather, his recent statement to GMA, was simply that he'd need Mark Hamill to be the one to tell him that he was wanted to play Luke Skywalker. Basically, he just wants the original actor's blessing. Stan's actual words were...
Well, if Mark Hamill calls me personally to tell me that he feels inclined to share this role with me, then I'll believe it, until then I won't believe it.
Certainly it's true that Mark Hamill doesn't have casting authority at Lucasfilm, but that doesn't mean his opinion wouldn't matter there or that he won't be part of the officially welcoming committee if and when such a decision is made. Of course, the likelihood of such a role being needed is the big question.
While the role is being called "young Luke Skywalker," Sebastian Stan is honestly too old already to play Luke prior to Star Wars: A New Hope. And I have no idea how one would create stories of interest in that era for Luke that didn't blow up the existing canon. The more likely scenario would be to follow Luke prior to the events of the most recent trilogy, and Sebastian Stan is the perfect age for something like that.
That's not to say that story even needs to be told on TV or in film, so there's no guarantee it would happen, but it would hardly be the first time that fan interest in a particular actor taking a particular role ended up manifesting exactly that happening.