If they do one, they should create an entirely unique storyline, rather than copying the games.
This.
And
I'd rather like a Fallout TV-series like Gabriel said, 12 episodes, 2 seasons.
Or it's shorter, takes place in the same area, concerning the main conflict and main factions, but it has one, or few, different main characters each episode. The characters in every episode are different and come from different backgrounds, but they are all shown to deal with the world of Fallout and the main conflict (for an example, think NCR vs CL-situation in New Vegas, but I'd like it original in a TV-series). Characters could be captured in one episode, rescued in another by the new main character, kill a character that was the main character five episodes ago, or team up with a few previously introduced main characters... anyways, thing is that in the end, all loose strings will be tied together, all main characters, that are still alive, will meet somehow and the main conflict in the story would have escalated to some serious degree and there might be some fights and rebellions and all that, and main characters would fight other main characters and, well, you wouldn't be able to tell who would survive the series and not. It would not be like any other series, like in Chuck, Chuck never dies, in Supernatural, the brothers never die, in The Walking Dead, Rick never dies (and I read the series, so I know that he won't for a while). This series would be more like... Harper's Island (which I have not yet seen, but I've heard/read about it) where people die in every episode till the series is over and it's only then that you know who survived.