Variety reporter Justin Kroll claimed on Twitter tonight that Crossbones -- played by Frank Grillo in Captain America: The Winter Soldier -- will reappear in Captain America: Civil War and serve as the film's main antagonist.
Kroll said it as an offhanded remark, as though it was widely known; it's been speculated, but not announced and Grillo has steered clear of confirming that he was even back for the film.
Beat by Marvel in that one, two things he will play villain but not main one, Crossbones still main— Justin Kroll (@krolljvar) November 14, 2014
When he says "beat by Marvel," what he means is that a number of industry trade publications discovered today that Daniel Bruhl had been cast in Civil War, but that Marvel reportedly elected to break the news themselves rather than respond to inquiries about the rumors.
In the story that immediately followed Civil War in the comics, Captain America was seemingly assassinated, seemingly by Crossbones. Both of those things later proved to be incorrect.