If you find a hole in that theory that actually makes sense, I'll give you a cookie. I don't think Bethesda wants to get sued for perjury by making a Fallout MMO, so it's pretty damn likely that it's an Elder Scrolls MMO.
And a possible hole I find in your theory is the use of the term IP. As I see it, Zenimax Online's website uses 'IP' in a plural sense (meaning many assets) as opposed to a singular sense - in which case it would have referred to a game universe and is the way you and others have interpreted it. In the sense ZO uses it, 'intellectual property' could mean any proprietary information such as graphic assets, sound assets, engine code, or other things and does not exclusively refer to using a game universe. I believe we can safely interpret Zenimax Online as saying through their website that since they're owned by the same company they can share game development assets with Bethesda much like they can also share assets with ID Software. Remember that Zenimax Online was formed and their website up before Zenimax Media bought ID Software, which is probably why ID isn't referenced there.
Allow me to offer my interpretations of ZO's website and hopefully put this useless MMO speculation to rest:
"...we have access to some of gaming's best known IP..." - If they were referring to a game universe then it seems more logical that they would have phrased it this way: "...we have access to
one of gaming's best known IP...", which ironically is how most here have misquoted it as saying.
"...the massive success of The Elder Scrolls IV: Oblivion and Fallout 3 only adds to the assets that helped fund ZeniMax Online." - The way they use 'assets' here (both monetary and IP) supports my interpretation of a plural understanding of IP.
"Because we are all inside the same corporate structure, we share IP and because our offices are only about an hour apart, we can work closely with Bethesda's staff." - Again, 'IP' is used in a plural form and therefore not referring specifically to a game universe.
I am not attempting to disprove here the theory that Zenimax Online COULD be developing a TES MMO, which is quite possible. However, some here have erroneously jumped to the conclusion that Zenimax Online's sharing of IP could only mean making a TES game - such a conclusion is pretentious at best with the information we have and at worst simply false.