WandaVision will take place after the events of Avengers: Endgame. Elizabeth Olsen and Paul Bettany reprise their roles as Wanda Maximoff/Scarlet Witch and Vision, this time on the small screen via the upcoming Marvel Studios Disney+ series. Plot specifics for the project remain tightly under wraps, but it's confirmed to be set following the events of The Infinity Saga.
After getting to know each other better in Captain America: Civil War, Wanda and Vision fully engaged in a romantic affair during the events of Avengers: Infinity War. Vision being powered by the Mind Stone made him collateral damage of Thanos' (Josh Brolin) mission to collect all Infinity Stones in the hopes of balancing the universe. In the end, the Mad Titan succeeded, plucking the powerful crystal from the humanoid's forehead, killing him in the process. Avengers: Endgame brought back all of those who died through Thanos' snap, but Vision remained dead considering that he died manually at the hands of the villain. Now, however, he's set to come back and reunite with Wanda for their upcoming show, though details of his return are still unknown.
Marvel Studios has taken the stage at San Diego Comic-Con Hall H, laying out their Phase 4 plans, including new details for WandaVision. The Disney+ exclusive series is slated to roll out Spring of 2021, with the premise taking place after the events of Avengers: Endgame.
It's not that difficult to imagine that Vision will be brought back to life. After all, he's a robot. There's also precedence in the comic books for his eventual return. The biggest mystery in WandaVision, however, is how. In print, he's revived with the help of handful of science and technology people (Hank Pym helped rebuild him during the comics' Heroic Age). Avengers: Infinity War posed the idea that he can survive without the Mind Stone, but it didn't explain how it can affect how he functions.
With the multiple timelines now established in the MCU following Avengers: Endgame's Time Heist, one potential route WandaVision can go is plucking an alternate reality version of the character from a different timeline and bringing him into the primary universe. How this can go about remains uncertain, but since Avengers: Endgame pretty much opened the floodgates with regard to time/space-hopping, it's much easier to pull this off assuming this is what Marvel Studios is planning to do.
In any case, WandaVision will allow Marvel Studios to fully explore the title characters, as a couple and separately. As two of the supposedly most powerful heroes in the MCU, they haven't been given the proper opportunity to showcase their abilities. It's an arguably more interesting premise setting the show post-Avengers: Endgame, since the studio can focus on their story without being weighed down with all the commotion and catastrophic end result of Thanos' hunt for the Infinity Stones.