When any movie franchise becomes popular it's going to draw in fans who dive deeper into it, and try to read more into it, than was ever intended. Sometimes you can get some really interesting theories that way that are a lot of fun, but don't expect them to get the official approval of the powers that be. There's apparently a theory going around that in the MCU, when Captain America went back in time, he became Star-Lord's grandfather. While that idea might appeal to some, James Gunn is here to tell you that it's simply not true. Also, the math just doesn't work out.
A fan recently brought this Steve Rogers/Peter Quill theory to the attention of James Gunn himself, who immediately shot it down. As Gunn points out, we actually see Peter's grandfather in the opening of the first Guardians of the Galaxy movie, and it's certainly not Chris Evans, even the old version from the end of Avengers: Endgame. What's more, Peter's grandfather is simply too young to have become a super soldier.
The opening scene of Guardians of the Galaxy takes place in 1988 and while we don't get an official age for Peter Quill's grandfather, Gunn explains in a later tweet to another fan that Meredith Quill was 19 when Peter was born, and that Peter's grandfather was only in his fifties at the beginning of the film. This would mean grandpa was born probably in the 1930s, making him only a kid during World War II. There's also the slight problem that if Captain America were Peter Quill's grandfather, I think it would mean the Guardians of the Galaxy movies are part of a branched timeline, and not the one where everything else in the franchise is taking place.
Of course, this theory does ask one interesting question, because while Meredith Quill might not be the child of Steve Rogers, it doesn't mean that he didn't have any. While this might all have taken place in an off-shoot timeline, if Steve and Peggy did have kids, one has to wonder who that child would be. Would elements of the super soldier serum have been passed biologically?
We know that the multiverse is going to become a factor in the MCU pretty soon, so we could theoretically see concepts from other universes still impact the one that we've been following for so long. Maybe the real reason Peter Quill can't be Steve Rogers grandson is that somebody else will be introduced in the future who is.