Well, they're doing their spin on the House of M comic book storyline, with a few other comic storylines thrown in for good measure.
The premise is: Wanda's been though the ringer in the past 4 movies that she's been in. She hasn't really dealt with the deaths of her brother (Pietro/Quicksilver - Age of Ultron) and her love interest (Vision - Infinity War) and, while a lot of people got a happy ending and had people returned to them in Endgame, she didn't. So the idea is that she's moved into a suburban neighborhood and, in her grief, created an alternate reality bubble where Vision never died and she has god-level control over everything in that reality, including the people that live there and including time itself. Either she created it, or it has been created for her by some crazy powerful cosmic entity. But it's in keeping with the idea that Wanda/The Scarlet Witch is one of the most powerful characters in the comics, and now, in the MCU. Each episode is set in a period of television history, which basically she has chosen. Every time reality comes creeping in, she rewinds time and changes it to her liking, and/or she expels the threat to her happiness from the little bubble reality that she's living in (Episode 3 - when Gabrielle/Monica Rambeau was thrown out of the bubble for mentioning the death of her brother at the hands of Ultron). S.W.O.R.D., the successor to S.H.I.E.L.D., is on the outside of the bubble, trying to get in to "save" Wanda. They think that somebody else is doing this to her, and they might be right to a degree. The character Agnes knows what's going on. You can deduce that for yourself. She's supposedly a character from the comics named Agatha Harkness, a Salem trials era witch that's tied to the Scarlet Witch in the comics and to the supposed cosmic entity that's theorized and hinted in the show to be behind what's going on. She's kinda hinted at him ("The devil's in the details" "That's not the only thing he's in". "Ralph looks better in the dark." etc). To a degree, Wanda knows what's going on (Episode 2 - "I'm not trying to hurt anybody").
Here's the cool thing: it looks like the stuff she created can survive outside of the bubble. When the black girl (who's actually Monica Rambeau, the little black girl from Captain Marvel, all grown up) was thrown out, her clothes didn't change. That could have huge implications, like Vision coming back to life in the proper reality, the survival of her children.
Now, here are the theories: The series is alluding to/introducing the big bad Mephisto (Agnes' husband "Ralph"). He's a Hell-lord from an alternate dimension in the Multiverse. We'll wait and see. He was part of the comic Infinity War series, and he's definitely a huge part in the creation of Wanda's children Tommy (the mutant 'Speed') and Billy (the mutant 'Wiccan') around the House of M storyline. Because we're getting Wanda's mutant children, that implies that we're gonna see the introduction of mutants in the MCU proper. So while we may not see the X-Men pop up here, the seeds are being laid for them to pop up later. Wandavision is basically creating the mutants in the MCU. And no, I sincerely doubt Hugh Jackman's coming back as Wolverine. He said he was done.
What we absolutely do know is that this series leads directly into the film Doctor Strange in the Multiverse of Madness, which Wanda's going to be in. And this series is laying the groundwork for playing in alternate realities, which is gonna have effects in the Loki series, as well as future MCU movies and series.