The Multiverse Of Madness
WandaVision may be weaving an elaborate mystery, but the wider overarching conundrum MCU fans have been trying to decipher has related to two of the coming Phase 4 films - Spider-Man 3, and Doctor Strange in the Multiverse of Madness. Both are confirmed to feature the multiverse in one form or another, and both also boast a throughline back to WandaVision, which has only added to the idea that Mephisto will have a big part to play going forward.
Elizabeth Olsen will reprise her role as Scarlet Witch in the next Doctor Strange film, while the Sorcerer Supreme will also be taking on a bigger part in the third MCU Spider-Man, which will unite Tom Holland's Peter Parker with Tobey Maguire and Andrew Garfield's, as well as those wall-crawlers' respective villains, on account of some presumed multiverse shenanigans.
One thing that is worth pointing out, though, is that in everyone's excitement regarding Maguire's Spidey returning (as well as Alfred Molina's Doc Ock, which IS rather special), we seem to have forgotten the main fallout from Spider-Man: Far From Home, where Quentin Beck revealed Spidey's secret identity to the world. Similar events played out in the comics, and it led to one of the most infamous events in Marvel history.
One More Day is a notorious comic (mainly because it's terrible), but it has been theorised that Marvel could be about to adapt elements of that comic in Spider-Man 3, by having Peter make a deal with Mephisto to wipe the public's knowledge of his identity.
In the comics Peter drove his bargain with Mephisto in order to save Aunt May's life, but the public also lost its knowledge of his identity after Civil War. The cost was his marriage with Mary Jane, which had been in Marvel's sights for sometime beforehand, and it's widely considered to have set Spidey's development back by a good number of years, if not decades.
The Peter of the MCU, by contrast, doesn't have a marriage on the line, but he does have his relationship with Michelle and plenty of other attachments Mephisto could use to his advantage, which also applies to the other live action Peters who are set to appear. It would also give Marvel with a way of telling more traditional Spidey stories going forward, with Peter's secret identity still intact.
Yes, it's all extremely nebulous and hypothetical at this stage, but you have to think that Marvel have linked these three projects together for a reason.