For a bring back America run, the only country I can think of is New Victoria since the U.S. will ally them to restore Canada, but the problem is that New Victoria doesn't have content and the alliance usually comes mid-late game.
For Nevada, the ones I can think is NCR, with Nevada not triggering Civil war and Twin Mothers, as it can form Colorado. I'm pretty sure nations like Last Patrol and Texan Brotherhood can join as well but I'm not entirely sure on those ones.

Didnt think about New Victoria. Yes Texas can join when its in the provisional government point in when doing a Nevada run. Its also the only one that can keeps its OWB tree in the Nevada path. Though NCR would be the first option you should look at even if they do lose their tree.