Personally I would love to set a game between Fallout 1 and 2, where the PCs are either agents of Tandi when she's in her mid to late thirties possibly early forties as she's in the process of forming the NCR, either hitmen or mercenaries selling their services to the person that offers the contract that pays the most, as it'll also give the PCs the freedom of who they decide to take jobs from, you get the point.
For the system I'd use GURPS, I've got experience using it and I've GM'd it plenty of times before in Fallout and GURPS was going to be the original system before they replaced it with S.P.E.I.C.A.L. (As every diehard Fallout fan knows at this point.) and it was also used for the internal tabletop campaign of Fallout the folks at Interplay did before they turned it into a videogame.
I've given serious thought to posting in the Games Tavern about a Fallout Gurps oneshot to learn the system for people interested, the three pre-generated characters would be the pre-gens from Fallout 1, Albert, Natasha, and Max, as they all incidentally represent what I would say are the three major archetypes in GURPS. Albert being the Bard, Natasha the Rouge, and Max being the Fighter to use D&D terms.
I don't quite have the inciting incident devloped, only that it'd be set after the Water Chip's been returned to Vault 13 and the location would be somewhere in The Hub, with the three getting themselves involved in typical Fallout Shenanigans. I might make a real game for it if people are interested after I'm done running my currant Call Of Cthulhu oneshot due to me just preferring to only run a single PBP game at a time.
That way I can devote my full time and attention to it so I can keep the writing quality good.