I know this isn't supposed to be 100% accurate lore-wise, but wouldn't the NCR make a better adversary than the Brotherhood of Steel? If I remember correctly, the NCR has states near Washington State (Like Arroyo, in Oregon). Under President Kimball and his predecessors, the NCR took on a more aggressive role as its resources began to dwindle, resulting in the occupation of the Mojave Wasteland. Considering this, it would make sense for the NCR to be pushing up into Washington State, and would make a good southern threat to the Yakima Republic. It could make for some cool dynamics when the reemergence of the Enclave comes into play too, since the NCR would probably hate them more than they'd hate the Yakima Republic.

I am mostly suggesting this as a replacement for the Brotherhood of Steel, since the Brotherhood is supposed to be (pretty much) wiped out at this point. Maybe the Brotherhood could exist, but as a group of ragtag survivors trying to lay low, and not a major faction. Maybe it could be made a mission to help the Brotherhood grow again, while the NCR tries to destroy them?

I also feel like maybe it would make a little more sense for the Enclave to be coming from the north. We know the Enclave on the oil-rig were destroyed, so I'm not entirely sure where they could have been hiding along the coast. Canada was annexed in the final years of the war, so it's possible that vaults were hastily built up there, and that the Enclave could have survived in them.

Just a suggestion, though.