In my opinion the theory behind the X6 was BMW trying to troll the world. They took the fantastic X5, reduced the headroom, decreased the cargo capacity, and lowed the roof line toward the end to make it a "sport-back". Then finally they raised the price by a couple thousand just to see if anybody would buy it.
Result? It sold like wildfire..
I think the X4 will be proof that history repeats itself, except this time it will be with the X3 instead of the 5.
At least its safe to say we'll know what the X3 will look like right? lol