The BoS backstory was a good idea. It's a shame that ShoddyCast couldn't do it...and basically just stopped making content, aside from a handful of very recent stuff. I thought about it myself, before they announced their plan.
Any number of vault concepts could be great as standalone horror films. They could also end with the survivors fleeing their vault, only to get captured by supermutants. Which would lead into a character focused, psychological thriller sequel of sorts.
The protagonist survivor would become a supermutant and lost most of their memories. Essentially you'd see the Unity cult from the inside, under the perspective of a brainwashed supermutant. Who is still coming to grips with their new self, while slowly realizing what they're really a part of. There would be two currents of change for this character: increasing understanding and radicalization.
Unbeknownst to the audience this would take place during the year of the Master's defeat. That way it can show the aftermath of the Unity, the scattering of members who are no longer under the Master's thrall. Just before that happens the supermutant reaches a tipping point in their indoctrination, and are about to do something unequivocally monstrous. The sudden loss of the Master, itself foreshadowed, causes them to question the action and delay it. Over time they start to regain a sense of individuality.
I suppose this could lead into another story, if in a post-timeskip scene they get captured for experimentation by the Enclave. Might be a bit indulgent, but there could be some interesting points of contrast. Namely unity v division, or just as an example of how strong identities can be just as much of a problem as the erasure of identity.
