Oh, yeah. Cap definitely exists and is available to be played. 

The reason I left the details surrounding him vague are so that whoever picks him up can decide on how he's "returned" to the present, that way nobody has to be beholden to the idea of him being entombed in a block of ice for 80 years. 

You could theoretically spin Captain America's origin in a number of ways, whether it's to make him a 95-year-old who aged in real time or have a series of people who succeeded Steve Rogers - if Rogers was ever even Cap at all in this universe. Or, if that's not your thing, you could do the origin story as it normally happens and work from there, based off of any pre-existing version. (Ultimate Marvel, MCU, ect.)

See, that's what I mean by flexible continuity. I'd approve an application for Diana Prince to have been this Earth's Captain America if the scenario and application are thought-out and sound enough. Or a reformed Red Skull, just to throw out as many ideas as possible. This is an Ultimate Universe, so there's nothing really set beyond the backstory beyond the loose concept of there having been a real Captain America whose existence was hidden by the government.

Speaking of which, in terms of any plans for any of that backstory, there are none. The backstory is where my part in crafting the game's setup ends and the player created continuity begins. So if anyone wants to do their own version of the JSA, The Invaders, Bucky Barnes, SHIELD, CADMUS, any of that, they're free to dabble.

So write your apps to your heart's content. The vetoing process of any details that seem off to the game's setting will only start once the OOC thread goes up and applications are put forth.