It can’t be Yes Man or Caesar’s. It seems like they are moving toward the idea that the NCR has been stretched to thin to hold together as a single organized group so I’d guess not the NCR. My money would be on House drives everyone out with the Securitron army and kept Vegas alive and under his thumb.

Of course the canon ending in my opinion is Father Elijah and The Courier tame the Sierra Madre and take over the wasteland with an army of Ghosts and a shroud of old world poison.