They'd pull a WoW and turn it into an MMO.
Stuff like this: http://www.bit-tech.net/news/gaming/2010/01/21/bethesda-working-on-mmo-similar-to-warcraft/1 is already pointing in that direction.
I hope they don't make a TES mmo. Even a Fallout mmo is a bad idea IMO. They're going out on a limb that it will do better than WoW and not flop like every other MMO that has came out. Plus SW: The Old Republic is coming out and I'm sure that's going to pull in a ton of fans. It just seems like too much of a gamble to me.
But when TES does end, hopefully it gives the series a truly amazing ending that it deserves.