Thought on the Prelude to season 9 episode

As we can see from this episode the Doctor is planning to go meet someone. Someone who he has known for a long time and who has always been an enemy. We are led to believe this is the Master yet from the trailers it seems the Master is the one seeking the Doctor, not vice versa. This leads me to think through who it could be and the best I can come up with is that the Doctor is being called by the Great Intelligence. The Great Intelligence I believe needs to Doctors help and as a result I think the GI will offer some sort of deal with the devil to find Gallifrey. Thoughts? If not the GI who? Any evidence you can see that I missed? Who is the Doctor supposed to seek after he cleans up a bit, someone who will most likely result in his death.