The Madara casket theory is okay, but I'm hoping its the Sage of the Six Paths. He was strong enough to seal the Jyuubi away, so that would TOTALLY wreck Madara's plan. From a strategic trump card perspective, that would do to get Madara to negotiate.

The First Hokage is an okay choice too, since the Senjutsu are supposed to be able to stop the Uchiha jutsu.