I would probably hire a different developer(no idea who) to start a new Fallout game from scratch, but enlist the aid of Obsidian in it's development (mostly for the story/scripting/quests department).
This is probably the best answer anyone could give.
Obsidian is a company with a better writing staff and some great creative minds . even mishaps like Alpha Protocol have great ideas behind them... The problem though is money... Obsidian are struggling and need backing.
Bethesda is becoming a mogul pumping out, until now, mediocre third party titles and huge exploration games.. Their own games are becoming more and more big budget, and less and less risks are taken to make them in financial successes.
So the choice is basically between: Great writers but financially unstable or a studio who is also a producing studio whose ambition it is to become like EA or worse Activision..