The Falcon and the Winter Soldier's big Episode 5 cameo got a major reaction from fans and hints at some big things to come in the Marvel Cinematic Universe. (Major Spoilers) The big actor in question was actually actress Julia Louis-Dreyfus (Seinfeld, Veep) who is playing the Marvel Comics character Contessa Valentina Allegra de Fontaine. As soon as Marvel Comics fans saw Contessa onscreen, the gears started turning about what Dreyfus' larger story arc in the MCU will be. Valentina comes with a pretty sordid comic book history, but there's one aspect of it that definitely seems to clearly connect to the story of the Black Widow movie.
In the comics, Valentina Allegra de Fontaine (or "Val") was one of the standout trainees at S.H.I.E.L.D., quickly catching the eye of Nick Fury, after she kicked his butt in a sparring session. Val and Nick were lovers for a time, but as it turned out, Fury (the elite super-spy himself) never should've let his guard down.
To make a long comic book story short, it turned out that Valentina came from a family of sleeper agents loyal to a covert organization called Leviathan. Leviathan was one of the organizations that splintered off from a collective of the world's eleven best covert operatives and secret society members, known as "Zodiac." When the group splintered, its members went off to form some of the key clandestine organizations of the Marvel Universe. Those spinoff groups included S.H.I.E.L.D., HYDRA, The Hand, and Leviathan, with the latter forming in the Soviet Union. Leviathan's leaders were actually the ones who betrayed Zodiac, by trying to steal the group's prize artifacts of an unlimited power source that can transform people into superhumans. Valentina Allegra played her role in S.H.I.E.L.D., then infiltrated HYDRA as the new Madame Hydra - but it was all a setup to help Leviathan rise again, and go to war with its rival organizations.
The introduction of Valentina Allegra de Fontaine to the MCU seems like a clear signal that Leviathan will be the true "big bad" of the Black Widow movie. Black Widow's storyline sees Natasha Romanoff (Scarlet Johansson) going back to Russia to investigate hints of a new super-spy program springing up there. Who would be in charge of that program, other than Leviathan?
It's sensible speculation: Julia Louis-Dreyfus was previously set to appear in Black Widow as Valentina before the COVID-19 forced The Falcon and the Winter Soldier to release first. It seems pretty clear that Marvel Comics' history is unfolding in the MCU.
Leviathan and its connection to the history of Zodiac, is really a story arc that seems to be taking shape across Phase 4 of the MCU. Nick Fury and S.H.I.E.L.D. are coming up in the Secret Invasion series, and The Hand could easily figure into the story of Shang-Chi (even if it's flipped into "The Ten Rings" organization). Valentina is the doorway to both HYRDA and Leviathan, and Black Widow can really get that ball rolling by showing how Leviathan was already scheming before The Snap and The Blip occurred.
Black Widow hits theaters and Disney+ Premium Access on July 9th. The Falcon and the Winter Soldier is now streaming on Disney+.