I wonder if Tsunade now has sufficient chakra to perform the Death God jutsu. With her being hokage, she would have access to the Hokage Scroll, and I would assume that it is there.
Um...why the hell would he want to do that? Did you not read the part where he is immortal and has unlimited chakra? Why would he give that up with Rinne Tensei?
I'm thinking this might be Kishi's way of jerk-tailing us around with keeping Madara alive during the DG jutsu.