Going off of Colonel Martyr's thread, I think this question right here is about as relevant as which New Vegas ending is canon. It's clear from New Vegas that the potential for the Great Khans to become a major faction IS there, the question is only if that ending will become canon.
So simply to clarify, who'd be for the Great Khans building an empire up in Idaho/Wyoming? I'd love it, cause wtf there's plenty of potential for war if we have the NCR, Legion, Great Khans and BoS (admittedly not at all a nation or major faction, but constantly a present threat) all within the core region.