This thread is dedicated to discussing WandaVision as it happens week by week. So only read if you are up to date
Currently just finished episode 5. I think I may have worked out the plot after the revelation of X-Men universe Quicksilver at the end. So obviously it seems like a huge revelation right now that Quicksilver has seemingly hopped over from the Fox (now Disney) X-men universe. I do think, however, this may be a misdirect. Wanda clearly didn't know that was about to happen, she didn't cause it to happen. So my theory is:
Someone is trying to make Wanda happy for a reason. They are trying to extract some form of power from her, potentially from her kids. It's clear she wants the kids to grow up by forcing them (killing the dog as a ploy to get them to grow older again). Her kids in the comics are Wiccan and Speed which have the same powers as her and her brother. I think it is either Agatha Harkness and / or Mephisto posing as Quicksilver from the Xmen universe, or they have actually plucked him from the other universe (and are manipulating him). I would tend to go towards the first option however. I think they tried to view the MCU version of Quicksilver to fool Wanda, but Mephisto/Agatha potentially got it wrong and viewed the Xmen version fo Quicksilver, and assumed he looked the same. This however has now opened up all the possibilites for multiverse and getting whatever Xmen characters they want going forward
As for how this all started in the first place: The new head of sword (forget his name but the generic looking grey haired guy), they are basically there to create and design new weapons. It looks like in the episode they even have Vision cut up into pieces, and use his body for research and creating new weapons and tech. Wanda was shown in this episode getting his corpse back and presumably taking it into Westview. Now, it was basically told to us that she cannot bring back the dead with her reality powers (this would be too OP, even for her). What I propose she has done (or maybe Mephisto/Agatha) they actually took the mind stone from another reality, and are using it to reanimate Vision with the help of Scarlett Witchs powers (the mind stone is shown briefly in a trailer clip). Now as you know from Endgame, the Ancient One states that you cannot take a stone from another reality and keep it, as this has disastrous consequences for that reality. Whatever reality she has taken it from (probably the Xmen one), is slowly causing huge issues for the multiverse, which will then in turn lead up to the events of Dr Strange 2: Multiverse of Madness, which she is also going to be a part of. Also, in the upcoming Loki TV show, it is the version of Loki that takes the space stone as well, this will probably make the problem even worse and accelerate the issue.
If you have seen the Mandalorian I think they are basically doing the same with this. The Mandalorian was used to seed loads of spin-offs and big world building events for Star Wars (examples of Boba Fett and Ahsoka upcoming TV shows). I think they are doing the same with WandaVision, and expect to see some other big reveals going forward. But also the small things they just mention (like them getting an expert in to create something to get through the barrier). This has a high probability of being Reed Richards (aka Mr Fantastic from the Fantastic 4). This will be seeded in WandaVision for a future Fantastic Four film in the MCU. Another is that Monica clearly has powers she is trying to hide for whatever reason. As the scan on her body didnt work and she refused a blood test. Assume this will be followed up in Captain Marvel 2
Anyway these are my theories / thoughts