I am currently working on a very large expansion for FNV where you will be able to cross the Colorado River and go into Legion territory. You'll also be able to go west from Outpost Mojave to places like the Hub and Junktown, south from Searchlight to follow the Colorado River further south, and north/northwest from New Vegas to go to places like New Reno, Redding, and Klamath. The expansion includes the entire Big Circle (New Reno, Broken Hills, Gecko, Vault City, The Den, Modoc, and Redding).

I'm trying to minimize the changes to the vanilla FNV world, so I probably won't have too much on the eastern side of the Colorado in the vanilla FNV worldspace. There is going to be a Legion camp on the east side of the river that you can go through to get to the eastern worldspaces. In my expansion, the Legion has a large fort at Kingman, AZ and is using an old pre-war prison south of Kingman as a slave colony to produce food for its army. Another worldspace will take you all the way to the Legion capital of Flagstaff. I'm tapping into the FNV end game so that instead of just ending the game after the battle of Hoover Dam, you instead get an additional quest to destroy the Legion fort at Kingman, Legion camps all up and down the Colorado, and eventually push all the way to Flagstaff and basically cut the heart out of the Legion. Conversely, if you play for the Legion, you'll be able to go the other way and push all the way to Shady Sands and destroy the NCR.

This is nowhere near ready to come out yet. There are seven worldspaces, six of which are complete on the exterior (the Big Circle is mostly done but still needs some work). Interiors and NPCs are done for a couple of the worldspaces, but very few quests and little dialog is done. So it will be a while before it comes out.