Well, Fallout complexities may lie in the character development.
Story and dialogues have some deep hierarchy and are based on a character's traits.
As mentioned, you need to keep track of a lot of variables because every player decision
will affect the story. Unless you're going for a more linear gameplay.
The turn based battle engine and the loot system are defying areas the will make your
game succeed or not.
I'd love to play your game if it's ever done.
Fallout 1 and 2 are ageless classics for me.