Thanks for the answers! With them in mind, I have a few ideas:

[list]
1) An NCR fanboy, now serving as a scout in the army. Originally belonging to a vault, he and his family were liberated from the ongoing experiment and assimilated into the union. The character and his fellow vault-dwellers would have revered the NCR, both for liberating them and for their resemblance to the familiar old world government they'd read and heard about from passed down stories. His father died in service out in Nevada, further fueling his tunnel-visioned admiration for the NCR. He'd be dispatched from an encampment in Wyoming (implied to be mostly tribal lands by Fallout New Vegas) to scout out potential alliances and solutions for an encroaching food shortage (Thomas Hildern, an NCR scientist in NV, expects a food shortage due to overpopulation about a decade after the game's events). 
Tag skills would be Energy Weapons, Barter and Survival. 


2) A former Enclave scientist from the Chicago outposts. A controversial figure on base, he fundamentally disagreed with the Enclave's philosophy: Rather than believing that the Wasteland had tainted humanity, and Wastelanders, Mutants and Ghouls ought to be exterminated, he believed the opposite—that this was a new path of evolution for humans to take, and that they should be researching how to perfect it rather than trying to exterminate it. A combination of his counter-cultural views and dubiously ethical experiments to prove them led to his ousting from the base. He's since wandered the Wasteland to further his research, seeking to unlock the key to humanity's next step and leading the remainder of civilisation into a new, golden age. 
Tag skills would be Unarmed, Science and Medicine. 
(Unarmed may seem unorthodox for a scientist, but I figured it could be fun to subvert the usual trope of Energy Weapons. Plus, it would make sense for his character to keep himself in top physical condition, as he believes in pushing humanity to its limits and beyond.) 

3) Originally a child soldier in the Legion, this character was driven out from Nevada by the NCR. Lead by a Centurion, he and a smattering of other soldiers fled into Wyoming, where they attempted to carry on the Legion philosophy as a tribe. Eventually, however, they would come into conflict with other enclaves of the Legion who disagreed with them, and suffered equally from internal squabbling. Gradually, the remainders of the Legion in Wyoming would break down, becoming more tribalistic and beginning to war with each other. Seeing no end to the conflict, this character chose to cut contact with his tribe: While they were in battle, he raided their tents for supplies and deserted them. Eventually, he ended up in Montana, where he now attempts to settle down and leave the scars of his old life behind.
Tag skills would be Melee Weapons, Survival and Lockpick.
[/list]

I figure I'll pick an idea based on GM feedback as to whether they're feasible, then by the balance of characters we already have as they start to appear—both to not repeat anything, and to offer interesting interaction and character dynamic potential.