I think we can safely assumed that the expansion is either going to include a new, smaller map or use the old map but in a different time period. Thus, an Eurasian-wide campaign map is going to have to be reserved for the next TW installment, and my guess is that it isn't going to happen. After all, good old Ghengis Khan 2 got so very, very boring after a while - with a super-simple strategic mode and an equally simple tactical battle mode, which cut down on time.

So my guess is either a new Viking Invasion or an American Invasion on a small Central/South American map - but I still regard that as unlikely, as that's going to mean only two european factions at the most. Perhaps they'll do something novel and use 15th-16th century Italy for a "French and Spanish Invasion". A continent-wide Arab/Viking/Magyar Invasion would also be fun, but the expansion of islam is going to be very oddly modeled if they don't expand the map to include all of Persia and India plus the arabian peninsula - which I don't think is going to happen.