Black Widow CAN return (spoilers)
Spiderman: Far From Home Nick Fury mentions Mysterio is "from Earth, just not ours. The snap tore a hole in our dimension"...this heralds the multiverse...meaning there are infinite universes that Black Widow exists in that she didn't die. They could easily write in that she crosses over from one of the alternate universes almost identical to ours where Hawkeye sacrificed himself instead of Black Widow.
Theoretically the same could apply to Gamora and Iron Man.