I think they’ll just open a time portal in the end of the black widow movie and avengers will say “we need you in the future” & boom, she’s back. According to endgame time laws that would not change anything she did in the past.
If you went back in time you would not ever be able to change anything. You would have to move time backward, not yourself back in time, to be able to keep your memories with you. Right? But then moving time would eventually either duplicate EVERYONE, and thus destroy everything that ever was and ever will be, i think