Well . If you are not afraid of spoilers in any way, know that Elisabeth Olsen has stated that the cameo that will appear within WandaVision will have the same impact as the appearance of Luke Skywalker in the finale of the second season of The Mandalorian. In other words, he would be a very well known character, important to the saga and loved by fans.
The possibilities are actually many. First of all, it could simply be Quicksilver, Wanda's brother who died during the events of Age of Ultron. We admit, however, that, even if it is a perfectly credible and sensible option for the story of Wanda, Quicksilver is not exactly a very famous character and of primary importance.
We speak instead of Magneto, villain of X-Men and Wanda's father in the comics. This hypothesis is a favorite with MCU fans, who would appreciate seeing the character enter the film saga. A third option, finally, is Doctor Strange who will appear together with Scarlet Witch in the sequel to the film dedicated to him, Multiverse of Madness. It is possible that WandaVision will serve as the starting point for the new film.
For now, these are only guesses, mind you, and there is nothing official. Finally, there has recently been talk of Eternals: the Marvel film is "huge and vast", it will surpass even Avengers Endgame according to the authors.
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