So many spoilers have said Madara's secret jutsu will have to do with a black rinnengan rod controlling either Hashirama or Obito.
shI say (in the light of the clearer explanation given by the lastest Naruto Anime release) that it could be:
Izanagi (he has the sharigan and the senju DNA); he might use this to create a realtiy of himself as being fully alive.
Izanami (he should know about this); he might wanna trap the whole shinobi world in the loop.
Sasuke's knowledge should therefore help them.
shI say (in the light of the clearer explanation given by the lastest Naruto Anime release) that it could be:
Izanagi (he has the sharigan and the senju DNA); he might use this to create a realtiy of himself as being fully alive.
Izanami (he should know about this); he might wanna trap the whole shinobi world in the loop.
Sasuke's knowledge should therefore help them.