I think we all know they are working on ES:V and have been since FO:3 was finished. They ain't talking about it becuase it would distract from Fallout Vegas coming out in the next few months.
It's too bad that Bethesada / Zenimax can't have a dedicated large world engine team, and then three or four game creation teams all using that engine. Could you imagine the piles of money they would be rolling in if they could release one massive RPG a year with a years worth of DLC following it? After the last of the DLC was rolled out for a game, they next big one would be almost ready for release.
I think they are working towards it. Well, Zenimax is, at least.
Think about it:
acquiring id gives Zenimax in-house engine development. Bethesda is "happy" with their engine, but they were also by many theories deep into content creation, and idTech 5 isn't "ready" for a TES-like game yet anyway (nor will it be until post-Rage, IMO).
Using the Bethesda Softworks publishing label, they can outsource (New Vegas)
Fallout 3 demonstrates that Bethesda has 1.5 teams that are able to execute well. If TESV is in fact the GI cover, then we'll even have the idea that this setup can roll 2 franchises effectively.
How do you grow your business from there? Expand the number of "partial teams" you have, while continuing to roll franchises. Evenutally, you have the critical mass needed to add a franchise.
What would the new franchise be?
Where would it fit in the marketplace?
Why that franchise?
How are they going to build a fanbase (very probably the most important question)?
Who will lead the new franchises (because a third fanbase is a bit much to ask Todd Howard to feed)?
When would they finally reach a point where new franchises make great business sense?