There already is a Fallout MMO. It's called Fallen Earth. Ok, it really ISN'T a Fallout MMO, but it's so easy to identify the scenery (desert wasteland scarred by nuclear conflict) that it might as well be a Fallout MMO. Plus the guys who made Fallen Earth are in this state and can have me killed at their whim, so I'm kinda holding back on saying anything negative about them.
As for what Bethesda is doing, my vote goes to Elder Scrolls V. I've heard it's going to take place in Skyrim (much as I want it to encompass the entire empire (Skyrim, Hammerfall, Vvardenfell, Sommerset Isle, Morrowind, Cyrodil, Black Marsh, any other location I forgot) and know that current-generation technology will not allow for it), and it will be set after the events of Elder Scrolls IV: Oblivion. Maybe they should call it Elder Scrolls V: Shadows of Oblivion since it's possible that with no emperor, attacks from the daedra could be quite common and that Oblivion, the realm of the daedric lord Mehrunes Dagon (Me runes, Dagon! "Oh shut up, Taven!" Sorry, lost my train of thought to Dagon being the name of a Mortal Kombat character.) may be casting a shadow of obliteration upon the Empire by first conquering its frozen reaches, Skyrim. Now, I'm curious...WHY WOULD ANY ARMY ATTACK THE COLDEST PLACE FIRST!? I mean, has nobody learned from Napoleon Bonaparte?! This guy starts his campaign against Russia, which may as well change its name to Really Fucking Cold Land, right at the start of winter and gets his ass kicked more by the weather than the Russians! And when did Adolf Hitler start his campaign against the Soviet Union? ON THE SAME FUCKING DAY AS BONAPARTE! Now, with Napoleon I can give a little leverage, he may have made a miscalculation or something. But Hitler?! I kinda thought they had invented meterology by 1939! Now, from what I hear, Skyrim might as well change its name to "What In The Hell Is Warmth Land" since the place looks like Bruma year-round. And it's full of mountains and natural deterrents, so why would the daedra attack, or even then why would the Nords LIVE, in this unforgiving, inhospitable place?! It makes no sense! It's like trying to live in Antartica! Unless you can survive by catching penguins, nobody in their right mind would try to do it!