If we did choose open, why? We need a reason for the PCs to stick together, at least in the beginning. Having it simply be a bunch of survivors meeting and agreeing without cause to just overthrow New Vegas would be hard to believe. Perhaps we could have a mixture. Some unknown, shady figure hires us to come together and overthrow New Vegas any way we can, or to eliminate the competition. Eventually we may come to disagree and fight for control, or betray our employer universally and work together to overthrow it.

Hell, you know what? Maybe we can scrap the New Vegas plot all together. Maybe we are hired to investigate a series of disturbingly powerful radiation spikes in a particular area and have to deal with whatever's causing it. Maybe some PCs aren't even hired, and simply show up on their own for their own reasons. For scientific discovery, for the promise of adventure, to vanquish something dangerous to us all, to scout out for Caesar or the NCR. Now that I think about it, that could be more open-ended and involve a little more violence. Don't get me wrong, I love conflict between characters. And we could certainly have it.