I'm not sure what more they can do with the Mojave. They'd have to canonize one ending and that will invariably make someone somewhere angry. NCR leave the Mojave in a relatively stable state and aren't sure great for a new adventure. The legion is an option as it most likely would have imploded in the coming years. The Mr. House and PC ruling are the easiest to go with as they don't change much in the overall power dynamic of the region. Plus, one man is much easier to kill than an army.
I'd bet that if they do make a New Vegas 2 it would have to be a prequel or it would have to been a hundred or more years post NV1.