Brock Rumlowbetter known as Crossboneswas played by the actor Frank Grillo. First appeared in film Captain America and The Winter Soldierthe mercenary was to play an important role in the narrative arc around Steve Rogers. However, from the first minutes of Civil Warafter a chase against the background of the recovery of a bacteriological weapon, rumlow commits suicide with an explosive belt by blowing up a building in the process. A very big mess of his character, while all the communication of Captain America: Civil War was built around him. If he returned at the bend of a fairly discreet cameo in Avengers: Endgamethere have been no more real projects around him while he has five projects left to do on his contract with Marvel.
However, the formalization of Captain America 4 worn by Anthony Mackie after the series The Falcon and The Winter Soldier could sign the return of the mercenary in several ways. Indeed, the official beginning of the multiverse after the events of No Way Home and Doctor Strange In The Multiverse of Madness could allow a new Crossbonesalways carried by Frank Grilloto join the Earth-616. Furthermore, Sam Wilson and Rumlow have severe adversity in Captain America 2 which deserves to be explored in greater depth.
Taking the character back in hand could allow Marvel to introduce a very threatening antagonist, well anchored in the down-to-earth and human-scale world of films Captain America.
The only downside, however: Given the formalization of Thunderbolts there is little and the very probable announcement of the casting which could fall during the D23 or San Diego Comic-Con 2022it seems likely that Sam Wilson instead crosses paths with a member of this new group and in particular that of John Walker, aka US Agent (Wyatt Russell). Marvel could give more credit to a newly introduced character rather than bringing back an iconic figure like Crossbones which, although misused, has had its day.
Nevertheless, rumlow could come back through flashbacks, to explain to Sam Wilson the strong roots of the organization HYDRA which would come back in force in the mcu thanks to Valentina Allegra de Fontaine (Julia Louis-Dreyfus). However, there remains a sacred unknown which also surrounds the character: the famous bacteriological weapon. Described as destructive, it boils down to a small vial recovered in extremis by Natasha Romanoff. The challenge of Captain America 4 could just as well draw on this history in order to reinforce the legacy of rumlow and make it appear by means of a flashback or a hologram.
It is, however, important that the character returns. Beloved by Marvel fans, Crossbones has sadly been underutilized in the MCU. Returning via the multiverse could allow him to breathe new life into it. In the meantime, Captain America 4 could know a synopsis at the next conventions where Marvel and Kevin Feige will be present.