Blue
Ravenous Bugblatter Beast of Traal
Wasn't blue serum what Winter Soldier took from the back of Howard Stark's car in Civil War? And was used to make the other Winter Soldiers? That's a solid character connection to Bucky as well.I believe that blue serum is from the Black Widow movie.
That's why I assumed this would come out after the movie rather than Wandavision.
I think that hand symbol is referring to Flag Smasher a villain from the comics.
Plus if we remember, the super soldier serum amplified what was already within you. Steve Rogers was that idealistic and everything and that got amplified, while Red Skull had his effect from the serum. So if that's in play it can have very different effects on who it gets administered to.