Back to the Future when Marty returns his name should be Dave.

In back to the future we see Marty’s actions affect future events, such as telling Goldie Wilson he will be mayor and playing Jonny b good.When he says goodbye to his parents he says if you ever have kids and one sets the rug on fire go easy on him. His parents laugh and his mom says she likes the name Marty.

In the future Marty came from he has an older brother named Dave. But in the new timeline he should have an older brother named Marty ,and he should be named Dave. When he asked about them going easy on their future kid, he subconsciously planted the idea of having kids in the minds of his parents ,and followed up with from his mom about liking his name would have linked the two ideas together. So when they have their first son it would be more likely that they would name him after their mysterious friend who set them up and instilled confidence in George to follow his dreams , than save that name for your third kid.

In the original timeline Marty’s name isn’t specifically said where it came from, so it can be presumed that they just picked it. In the new future, their is now inspiration from a person for the name. Thus ,it’s most likely that they would name their first son;who would be born closest to when their memories of Marty are still fresh and pick the name Dave for their second son. Ergo Marty McFly should be now Dave McFly.

Edit: just rewatched BTF3 Marty runs into his ancestors in 1885. He had a great great uncle Martin McFly that got killed after being called chicken. If anything Marty is the family name, not Dave.