Absolutely not. The day an MMO is released for TES, or even Fallout, will be the end of of this epic franchise.
Look at Blizzard. Diablo III was rumored to come out years ago, and so was Starcraft 2. Well, Blizzard hopped on the MMO bandwagon and made this game called World of Warcraft. You might have heard of it. It's supposedly a big deal. So much time and money was invested in WoW, that D3 and SC2 quickly became things of the past.
If Bethesda made a TES MMO, we'd never have another TES singleplayer game. If Fallout Online is made, then Bethesda will shift their attention and budget to that, thus putting TES VI on hiatus until further notice, possibly permanently.
Would it be awesome to travel all of Tamriel with friends? Yes, it would. The only problem is, MMO's are NOT story driven and you can't get immersed when an NPC is telling you, "Thank you *insert name,* for killing those spiders. Here, take this one and only bow from my armory. There's no other like it!" Yet, 300 people just did the exact same thing and got the exact same bow. Takes away the feeling that you're doing something epic.
I don't condone a TES MMO, but the only way it would work is if each race had it's own main story line(and started in their rightful province), and quests were randomly given. I dream of an MMO where a friend and I go to get a quest from an NPC, and we both have two entirely different quests. One where the reward I get is literally the only one of it's kind. This can be done. Diablo randomly generates dungeons and items. Ultima Online randomly generated items found on mobs. So the whole random quests and rewards can be done.
Also, like someone said, PvP would be horrible due to self crafted enchanted items.
So, in short, no