After introducing Brock Rumlow in Captain America: The Winter Soldier, Marvel Studios ended up wasting Crossbones in the first ten minutes or so of Civil War when he was seemingly killed by the Scarlet Witch after trying to blow himself up. This is the Marvel Cinematic Universe we're talking about, though, so it's entirely feasible he may have somehow survived and will return to plague Steve Rogers.
Despite stating in a previous interview that he's done with Crossbones, Frank Grillo has now dropped some hints about what comes next for the villain and it sounds like big plans are being put in place!
"It’s great to be part of Marvel too," he says, "and I have three boys who are fanatical about Marvel, and I was surprised not too long ago because there is some news that nobody knows yet, to do with my character, that’s going to be really interesting and I’m excited about that." Avengers 4 seems unlikely but there are other Marvel movies he could appear in, including the 1990s set Captain Marvel!
We'll just have to wait and see but feel free to share your theories in the comments section below.