Dr Fallout
Centurion
A game about the fall of the Legion could be fucking spectacular! You'd have a bunch of fanatics all emulating historical/mythological characters (like Spartacus, Romulus/Remus, Augustus Etc.) that they don't actually have an understanding of all warring against each other for control of the Legion; meanwhile the army slowly falls apart as they regress to their tribal ways and fall to the mercilessly expanding NCR.
Maybe have the NCR weak and trying to withstand the rampaging Legion. You know... please write your own ideas on what would happen. It seems really open and full of ideas.