Well ... ya know they could sell the MMO trademark rights w/o selling the TBS or RTS (or anything else for that matter) trademark rights....
Although it is unlikely that a company would willingly buy only the MMO rights if they know the company that sold it could make another game which might draw from their MMO crowd ...
But then again, an MMO is a completely different type of product than TBS/RTS/Book ... so it would be perfectly acceptable to have the True Canon (Stardock's Elemental and subsequent releases based in the Lore)
and the Blizzard spinoff Canon (For their new MMO...)
So yea, I'd be willing to sell *part* of the Elemental franchise (if only the MMO part) if I was Frogboy ... however never the TBS cause that just isn't Blizzard's realm of expertise ... and I wouldn't want them to sully Elemental's good name.