Admittedly, announcing that you'll have a didn't-see-that-coming cameo kind of defeats the purpose of even having a cameo in the first place, but whatever.
When asked by TVLine if 'WandaVision' has a cameo up its sleeve ala 'The Mandalorian' and its digitally de-aged version of Luke Skywalker, Elisabeth Olsen responded in the affirmative and stopped short of giving any further details. Showrunner Jac Schaeffer was equally tightlipped, simply stating that there are "many surprises left in store" for the remaining five episodes.
So far, the show's had a couple of supporting players from other characters in the Marvel Cinematic Universe. Jimmy Woo, played by Randall Park, was previously in 'Ant-Man & The Wasp', while Dr. Darcy Lewis, played by Kat Dennings, was in 'Thor'. Now, both of these actors have chops when it comes to sitcoms, so odds are both of them were brought back on the basis of that.
Other than that, there's been no other Marvel characters appearing on screen. That said, there's been plenty of nods and winks to the wider universe, especially in those adverts that play between some of the sitcom episodes - such as a reference to Stark Industries, and HYDRA.
All of which leaves us with the question at the top - who's it going to be? Given that Vision is dead and was brought back to life inside the creepy town where the show is set, it could well be that Wanda is able to bring her brother Quicksilver - played by Aaron Taylor Johnson - back to life too? Or it could be something really obvious like Thor, Captain America. or Iron Man.