A fractured California would be cool. Former NCR states (The Hub, Boneyard, ect) become competing city-states. Or maybe California splits into new political entities, some of which may control some of the former NCR states.

You could open up the Mojave too. Newly-formed nations that formed in the wake of the conflict there. Settlements like Primm (with its strategic position along a major trade route) that have grown to become significant powers. Former NCR generals or Legion legates with fantasies of ruling as kings form small dictatorships. Confederacies of settlements banding together to deter further military occupations, while also holding dear to their independence. 

These ideas depend on both the NCR and Legion collapsing, though. Which isn't hard to imagine, since the NCR is desperately low on resources and corrupt, and the only thing keeping the Legion together is Caesar. If he dies, the Legion falls into civil war among legates.

The areas further south in Mexico are interesting too. NCR troopers hinted at an NCR Ranger presence there, saying the Rangers are "chasing ghosts", which could mean anything from a special kind of mutant, to the remnants of the Enclave.