Well, the Skull was the one who gave him the name Crossbones, because he intended for him to be his sidekick or whatever. I guess they could introduce Crossbones without the Skull being involved, but that would be sort of strange if Cap keeps running into villains with names that reference bones.
ATTENTION: The rumor about WB's "no jokes" policy is 100% correct. No one should ever doubt any rumor about any upcoming movie, even in cases where it contradicts rumors that have previously been reported. In those cases, both rumors are true.
In other news, Bryan Cranston is playing Lex Luthor, Emily Blunt is Wonder Woman, Benedict Cumberbatch is Martian Manhunter, and that guy from Teen Wolf is Batman.