There are few things more annoying than a prequel. Even if it’s a character that I care about, prequels lack the necessary tension and drama. Instead of being worried whether or not a character will be able to survive his or her trial, the outcome is already certain. There’s no reason to be worried that the character will die or face other drastic consequences. Black Widow and Scarlett Johansson deserve better than that. Unfortunately, all signs point to the Black Widow movie as being a prequel. After all, Black Widow is dead. She traded her soul for the soul stone and that trade was “an everlasting exchange.”
Why and how would a stone require a soul? In order for the information on the deal to be transferred to the Red Skull and enforced by the stone, the stone itself must have a consciousness. It must be a living entity. Unless Vormir, the stone’s home, is the living entity that guards the stone.
If the stone is a living entity, it is clearly evil. It desires a living sacrifice in order for others to access its power. That soul for a soul exchange and the disposition of the stone could be the subject of the Black Widow movie.
The stone could just require the soul to power it up. However, other versions of the soul stone have included a soul stone universe where all the souls that were sacrificed reside. Black Widow could realize she’s trapped in an alternate universe and work to find and fight her way out, possibly with the help of Gamora.
It would be an interesting story line to explore and with obstacles that Black Widow would be more than capable of overcoming. Imagine Marvel’s next villain being the Soul Stone Irregularity. Of course, the Black Widow movie doesn’t have to take place in the current MCU at all; the multi-verse and alternate timelines open up so many story telling possibilities.