My theory is that WandaVision will cause the barriers between worlds to weaken and that's how Nightmare gets to do his thing, since he's the main villain.

^ Nightmare is not the main villain; in fact, he may not even be the villain. C. Robert Cargill & Scott Derrickson considered him for the sequel, but neither of them are involved with the project; in fact, Cargill even confirmed that didn't even submit a draft for Doctor Strange in the Multiverse of Madness. Whatever we get is by Sam Raimi & Michael Waldron. That means everything Derrickson, Cargill and leakers revealed based on the original story idea went out the window the moment Derrickson and Cargill left the project.

Oh I missed that update. Either way, whatever villain it is will be empowered by weakening between worlds caused by WandaVision.