WandaVision Episode 8 ended with the post-credits reveal that SWORD has rebuilt Vision's physical body - which they now presumably control. However, Marvel fans freaked out hard when they saw that this new version of Vision is none other than the iconic White Vision from Marvel Comics lore. Vision's white body first appeared in an old Avengers West Coast storyline from 1980s Marvel Comics, which has been influencing WandaVision in some key ways. However, that same storyline, "Vision Quest" ultimately revealed that Vision being disassembled and rebuilt in his white body was part of a plan by a larger big bad - and WandaVision may have just confirmed the same!
In "Vision Quest," it is rogue agents of the US government that abduct Vision and dismantle him, rebuilding him in a chalk-white body. While Vision is able to be brought back online (mechanically-speaking), the rebuild leaves him devoid of his "soul" (i.e., the brain patterns of the late Avenger Simon Williams/Wonder Man). The White Vision made its debut by attacking Scarlet Witch and his fellow Avengers, ultimately settling into a strange new existence where he was much more machine than man.
Clearly, that kind of major conflict (and its ultimate resolution) is possible for WandaVision's finale episode, as the "soul" of Vision Wanda created inside her Hex bubble reality has to face his physical white body. However, the key detail to the "Vision Quest" story is why the government agency took Vision apart and rebuilt him in the first place. It was all part of manipulations by Immortus, the would-be master of time and the multiverse. With WandaVision revealing White Vision as SWORD's unholy creation, it only adds the most credit yet to our long-running theory that Immortus is actually the one pulling the strings as the true big bad of WandaVision!
If you haven't read our previous breakdown on WandaVision's big bad villain theory, here's the long-short of it: Immortus is pulling the strings of SWORD to affect events in Wanda Maximoff and Vision's lives, in order to ensure certain cosmic events come to pass. Wanda's existence as a Nexus being capable of power chaos magic made her an invaluable tool for Immortus to change time, reality, and probability in ways to suit his own designs. The rub was that Immortus was constantly locked in a chess game with the greatest threat to his master design: himself.
Immortus is the elder version of Marvel villain Kang The Conqueror - a warlord who hops between timelines and realities, conquering them one-by-one to establish his temporal empire. Well, Kang the conueror has been cast for the Marvel Cinematic Universe (actor Jonathan Major), and it's long been the presiding theory that Kang will be the MCU Phase 4 big bad. The connections get deeper as Kang's younger self becomes a superhero (Iron Lad) who travels to present day and forms a team of Young Avengers. That's another Marvel franchise that WandaVision seems to be helping set up, with the introduction of Wanda's sons Billy and Tommy, and Monica Rambeau getting her powers as Photon/Spectrum.
The delicious part of that villain choice is that most mainstream fans won't be aware of an Immortus/Kang connection, so this connection between WandaVision and the other MCU Phase 4 projects we know feature Kang (Ant-Man and the Wasp: Quantumania, and the Loki series) is one that Marvel Studios could milk for a long time before it all comes together.