A resurrected Black Widow!

Natasha Romanoff definitely died in Endgame. But if you wanted to emotionally devastate your existing characters by briefly reuniting them with their mentors/true loves/bffs, the multiverse opens the door. This is a power to use very sparingly: nothing diminishes the power of a death, and with it the emotional weight of a franchise, than endlessly resurrecting people who are gone, or permanently importing replacements for them. Just, please, close it again and let the dead lie.