Captain America is a very rich concept. A solid Golden Age creation of a true-blue all-American warrior, he has become a mainstay of the Marvel Universe, a core character central to the mythology. That mythology projects both into the past and into the future. Captain America has been re-imagined in the future several times it seems.
Rich Buckler's creation Deathlok was originally intended I think to be a Cap of the future, a hard-bitten character dangerous and deadly living in a rugged and bleak world, he was a Captain America with a really potent gun and a big chip on his shoulder. For a sidekick he didn't have the rambunctious Bucky, but the stolid "Puter". I don't know if I've read this anywhere, but he's always been a Cap on the dark side for me.
Another is Vance Astro, or Major Victory. Arnold Drake and Gene Colan created him to obviously be a Cap figure of the future, finding the shield ultimately under the hand of Jim Valentino, and naming the Guardians' ship the "Captain America" before that. Clearly Vance Astro, finding himself a freedom fighter in the distant future on an Earth overrun with invaders from space, and he needed and wanted to call upon his 20th century hero to inspire the overall effort and himself.
And finally there's OMAC. The One Man Army Corp is a creation of Cap's daddy Jack Kirby and by all accounts was originally a Cap-of-the-future concept that Kirby ended up using at DC instead of Marvel. The world of OMAC is a dystopic one as well, more akin to Orwell's vision than anything else, as OMAC is a servant of a corporate world run amuck toying with the very definition of what it is to be human.
So here are three Captain Americas of the future, all in somewhat dangerous times and grim places. All find themselves in worlds needing a Cap, but all worlds need that really.