Currently Online 
It ought to be TES V. I'm pretty sure they're not making a new Fallout since Obsidian is already working on it and I'm not sure Bethesda would want to work on a new IP.
That said, Gamebryo is a horribly buggy engine... And it doesn't even look very good. I really wish they got rid of it as quickly as possible. Anything else would be better. I'm sure id Tech 5 could work very well.
Anyway, hope it's a more dynamic world and I'm not speaking about enemies. I'm talking about events and actions. How cool would it be to see a party of treasure hunters go seach a dungeon in hopes of finding something valuable. Or when the players saves the world again, it would really mean something besides some occassionally shouting 'Hail, Champion of Cyrodiil!' or something else like that. And AI... I always hated it when a bandit attacked me and if there was a Legion soldier nearby and I ran to him, the bandit just followed not caring about his life.
