I very much doubt we will see a Fantastic Four origin story in WandaVision if the FF are going to get their own film.
I expect the refrain of "oh, geeze, not another origin story" has been heard a lot at Marvel. I think they might well explain the origin, but not bother to make it "a story" in and of itself. For some characters, the process of the origin is itself very important in establishing who the character is. For others, it isn't. I think the FF, and Photon, sit in the latter space, and they can be given a quick justification here without damaging the later story.