As long as they're not trying to shoehorn Harold or Dogmeat in again and nobody mentions Little Lamplight, I'll probably be able to keep my cool. Anyone's a possibility, as New Vegas only takes place a few years after 3... It's sad, but there really weren't a lot of Characters with a capital "C" in FO3, so one's just as good as the other. If they wanted to be cute, they could give us someone like Eddie DeLoria or Ben Canning, but what'd be the point?
Some pointless speculation, provided there really is going to be a "crossover" character (and, a-durr, here there be spoilers):
*The Brotherhood could likely furnish a character, as has been said. One interesting storyline hook (though you wouldn't know it from the way Bethsoft handled it in 3) is that the Eastern Brotherhood is currently in custody of the last surviving Maxson.
*Linden and his Outcasts have all along been operating with the stated aim of eventually reconnecting with the Western BoS.
*Mister Burke, the man-behind-the-man at Tenpenny Tower. It's a longshot, but his shadowy motives never seemed to be fully (or even partially) explored, and in the hands of a competent storyteller, he could figure well into a morally grey cloak-and-dagger excursion similar in tone to Dead Money.
*Three Dog might be one of the first characters to spring to mind, but his focus and expertise seem to be the DC Wasteland, and I don't envision him seeking to change that.
I can't think of too many others that wouldn't be utterly pointless or wouldn't grate with the tone that New Vegas has made such great strides to reestablish. Harkness? Hopefully not. Fallout isn't Blade Runner and it isn't Star Trek, and the presence of androids clashes painfully with the retro-future aesthetic. Stanislaus Braun could be a compelling presence if he were intentionally or inadvertently admitted onto the vault-tec Network at large, but he'd amount to nothing more than an evil House unless handled just right, and he raises too many questions (House survived for 200 years by investing hundred of millions of dollars in the necessary research and technology, and Braun did it by... strapping himself into a chair?).
All in all, I must say I'm a little apprehensive. Braun v. House is a perfect example of why... the logical divide between F3 and F:NV is nearly insurmountable, and their tones are irreconcilable, if F3 can be said to have a tone at all. There were very few places where New Vegas touched on F3 at all, and it was the better for it. I've often thought to myself that I don't envy the dev team that's going to have to hammer out a game where both 3 and NV are considered canon.
