The thing about expanding Legion "civilian" content is that... it would be quite hard in terms of quest content, especially with NV's engine. Imagine Cass' quest translated in a Legion controlled territory : it would simply be an investigation, with zero action, and the engine is... very limited when it comes to provide strictly investigation content.
I mean, neither Alice McLafferty nor Van Graff would have been able to conspire the way they did without the legion putting them on a cross, since anything that doesn't contribute to the common good is seen as a threat to society for them ; playing both sides in a war for your own interest would mean only one thing : death.
Cass wouldn't have to move her western butt away from the bar to get the justice she's after. She'd just have to hire an investigator to provide proofs of what happened, and the state would take care of the rest. It would be fascinating, story wise, but in terms of gameplay, it would require a little bit of GECK gymnastic, the kind that was impossible to do during NV's short development. I mean, quest markers pointing you to the clues you're supposed to find by yourself ? I'd expect something a little better.
Many usual conflicts would be hard to translate in a legion controlled territory, since it is defined by a very hard, yet super effective justice, the kind we simply never saw in Fallout. Regular players would wonder when they can shoot raiders and snort Jet, and the answer would be : "er, never. Things are different, here. No drugs, no raiders, no weapons in cities."
The only way to provide conflict would be to have the player be part of some sort of resistance trope, like being part of a faction who wants either Vulpes, Lucius or Lanius to be named dictator perpetuus after Caesar's death. Could be fascinating, but that's an entire new game we are talking about.
Or basically Tyranny translated in the Fallout universe. Being part of the Frumentari and provide justice in the way you see fit.