I've just begun reading OMAC: One Man Army Corps by Jack Kirby, so I don't have any significant thoughts yet. I am intrigued by a fact thrown out by Mark Evanier in his introduction.
He says that the original concepts used by Kirby in the OMAC series was developed as a future version of Captain America, which is interesting in a number of ways.
But I found myself thinking, and I'm unable to think of a preceding series that focused on a future version of a character. There had been "imaginary" appearances here and there, I believe, and we've certainly seen plenty, such as Spider-Man 2099.
Mind you, there's remarkably little excitement generated in determining yet another idea that Kirby had before everyone else, but it seems potentially like an interesting addition to the (very long) list.