It may be wrong to say that the Diablo franchise has its biggest fanbase in Europe. However, I think it could be correct to say that in Europe, the Diablo franchise is bigger compared to Starcraft and Warcraft than it is elsewhere. At least that's the impression I'm under.
If they're going to announce something in Europe, a Diablo game is more suited to that than any other of Blizzard's franchises.
There's the dates and the site acquisitions. For the record, I was just saying that because you were dumbing down the whole thing to one object of attention, and, for the record, I'm not saying I either support or do not support your theory or the WWI or BlizzCon theories.