In my version of the Multiverse Saga, Black Widow wouldn't actually change at all. I really liked this movie as-is, and finally paying attention to the dinner scene at Melina's farm makes me like it even more. If I gave myself omnipotence, I'd have tried to release it before she died, either right after Civil War or at least some time before Endgame, but that's it.
Sorry, fac.
I do really like fac's idea of having it all take place between IW and Endgame. That's a pretty smooth alternate idea that would fill in a lot of what we should've seen re how bad the world was post-blip.
I'm not the type to steal ideas, but I'm not against a group project, and could see myself voting for this in an Articulaverse MCU.