Three thoughts about this. Where the Devil-Mouse is going with WandaVision is pretty clear.
First, they're laying the groundwork for X-Men and Fantastic Four coming back into the fold. The cat's already out of the bag Kang is going to be set up to be the MCU's next Thanos, which means they at least need to fast track FF's entry into the MCU. Putting Agatha in WandaVision is a dead giveaway for it since she was historically closer tied to FF than Scarlet Witch herself.
First and a half, bringing in Kang is fucking stupid and there's one reason they'd do this (but more on that on point three). The far cannier play for phases four and beyond would have been to bring in Doctor Doom and the Masters of Evil, where instead of The Avengers 5 being the tie-in movie, do Masters of Evil where phase four and five's big bads team up. Half the Masters of Evil are already in the damn MCU, and the other half already have ground work laid for their introduction.
Doctor Doom would have been perfect to introduce at this point. He's a sorcerer in power armor, when Doctor Strange just ate a big fat nerf having lost the Time Stone and Tony just, you know, died. Doctor Doom would have been an ideal foil for Strange, being a serious contender for Sorcerer Supreme against a (somewhat) depowered Strange, while simultaneously rubbing salt in fans' wounds by being an evil Tony Stark. Then bring in Fantastic Four to stop Doom.
Second, it ain't about the damn X-Men. This isn't going to be House of M like most of us thought it would. Back in 2014 Marvel Comics retconned Scarlet Witch, so she was "never" a mutant to begin with and neither she nor Pietro were related to Magneto; Wanda was "always" a witch (that whole stupid Nexus Being shit), and if I remember right she's the one who gave Pietro his powers. Now given the time of the comics retcon it was pretty obvious it was a play by Disney to shore up Scarlet Witch's role in the MCU and avoid lawsuits by Fox, but now that risk is over Disney seems to be looking to tie Wanda more closely with FF than X-Men.
Third, this is about setting up the Young Avengers in the long run. Disney pretty clearly learned their lesson bringing in actors in their late twenties and thirties to play roles for a decade or more, especially ones with established names with more bargaining power than Disney is entirely comfortable with. Devil-Mouse wants an MCU roster it can lock in at an early age, and before actors have name recognition beyond the MCU to leverage for better contracts. And all these Disney+ shows and phase 4 movies are linked to Young Avengers characters, however obliquely in some way.
...and that's why Kang is the next Thanos. Because Kang is Iron Lad.