Hey howdy hey!
Needless to say, SPOILERS AHEAD. You’ve been warned.
In Endgame, the “leftovers” went back in time to retrieve the Infinity Stones. Black Widow and Hawkeye went for the Soul Stone. After a skirmish to determine whose life is saddest and should die, Black Widow kicks it, allowing Hawkeye to get the stone.
But remember just prior, Red Skull explained “a soul for a soul”, meaning a sacrifice in ordee to get the stone. Black Widow then was the exchanged soul.
So what happens when Captain America brings the stone back? Other than the awkward meet-up with Red Skull, in theory the stone could only be returned if it was swapped for a soul. Again in theory, thenmost recent soul would then be returned - Black Widow.
Mind blown, right? She could have come back and hid away so as not to upend the timeline until it was safe to come home. So there’s my theory.
Happy trails.