What if, correct me if I am wrong, but what if Ansem SoD and Xemnas are in the world of sleep? And that Master Xehanort knows this and tries to get them out by opening the "Keyhole of Sleep"? It would fit in with Yen Sids theory that there might be more than one Xehanort. It's possible that Yen Sid knows this and is why he said to Sora and Riku that they will be masters if they return.