Now, depending on which source you choose to accept there are 3-5 Old Gods on Azeroth (we know there's at least 4 now).
Obviously we know C'Thun, Yogg, Y'shaarj and Nzoth.
However, what if there actually was a 5th, and he was the most scheming of all.
Once we deal with N'Zoth (which I imagine we will within 2 years) that will be the Old God threat all but eradicated, unless something major happens to free the defeated Old Gods, which is a bit sad.
What if, all along, we'd been getting corrupted by a 5th Old God.
We know C'Thun Yogg and N'zoth were once at war and then chose to make an alliance, what if a 5th God existed somewhere on Azeroth and he had been using us all along to fight his war against that alliance and he will only make his presence felt once his competitors (the 3 mentioned before) were killed/beaten down for a long, long time.
It's all speculation obviously, but I think it'd be an interesting plot point to have one who has remained hidden and separated all this time plotting his master plan.
EDIT: By final I obviously mean on Azeroth, it appears there's 1000s of them in the universe