MCU: 10 Possible Uses Of The Multiverse In Phase 4 (And Beyond)
7. ...Or Just As A Way To Feature Them In Other Realities
Alternatively, those dead characters don't even need to be brought back to the main MCU timeline if the multiverse is involved. Sticking with the Black Widow movie again, what if it's just set in a reality where Natasha is alive and well?
In a recently-released clip from Spider-Man: Far From Home (below), Spidey meets Mysterio/Quentin Beck, who tells Peter that there are "multiple realities", including the main MCU reality (Earth-616), and Mysterio's alternate reality (Earth-833). Mysterio then explains that these realities have "identical physical constants, level four symmetry", which basically means that some realities out there are the same as the MCU we already know. Same Earth, same atmosphere, and likely the same people.
Of course, the people within these realities may be at vastly different points in their lives - for example, the Quentin Beck of Earth-616 probably isn't Mysterio yet, and maybe he never will be - which means that there could be a world on which Black Widow is still alive and well.
From here, it would be incredibly easy for her solo movie to just jump to that reality and set the story there, as opposed to bringing that Black Widow back to Earth-616. The Black Widow of Earth-833 (pick any number, really) could even be identical to the one we've known since Iron Man 2, which would make it a little easier for audiences to jump right into her movie.