This week, Comic Book Movie listened in to the radio show of Matthew Aaron, who appeared to have an inside track on the whole thing before it was a done deal. Aaron had Grillo on--and while Grillo said very little, CBM says that after the actor left, the host continued talking about him, saying that he had heard from the same sources who originally sold him on Grillo's involvement that Crossbones will appear not just in Captain America: The Winter Soldier, but also in Avengers 2 and a third film, probably Captain America 3.
The comments thread, of course, immediately leapt to the obvious conclusion--that if a character like Crossbones was to appear in Avengers 2, it would logically suggest a version of the Masters of Evil. Those characters have long been suggested and/or suspected by fans as potential villains for the sequel, although it's difficult to guess exactly how that would jive with the soft confirmation we already have that Thanos will be the big bad.
A supervillain team being put together so soon after the formation of The Avengers would also almost certainly lead to raising some of the same questions that Christopher Nolan did at the end of Batman Begins--does the mere existence of costumed superheroes lend itself to criminals that take on the tactics and fanciful style of those heroes?