Union Jack would be an interesting and amusing choice, although in terms of the story from the whole series to this point I'm hoping it'll be Bucky. He maybe seems too obvious, though...

If for no other reason than that he has no costume, it may have to be Bucky. Union Jack is british, and lets face it has one of the coolest costumes ever,so he's a no way. Ronin/Hawkeye has two costumes, and probably the second coolest costume ever, so he's out. Bucky is just too obvious, so I think its the Red Skull, afterall why else put him on that poster at all?

its gonna be batman.

It's probably Hulk. Actually I'd have preferred it was someone new entirely, why just use one of the old characters? A new character means they could really do whatever they wanted with him