Our main old antagonist Orochimaru consumed a good half of the focus of the series, and I deep deep down really, highly doubt that we have heard the last of him. There a many interesting possibilities concerning Kabuto's sixth ressurection. Jiraya, Madara's brother, and Senju the first Hokage would all be powerful, interestig and compelling candidates that would make for a good plot twist. However, we have to judge our hypothesis from our one most important peice of evidence: Madara's horrified and shocked reaction, accusing Kabuto of being insane. One as ominously as powerful as Madara wouldn't be quite as awestruck from the power of another being, but moreso by the value that person once represented, and the connection they have to either Madara himself, Kabuto, and/or the Shinobi world. Izuna (his brother) would make for a good candidate, yet has since then been far surpassed in terms of accomplishment and ability. Senju is a perfectly logical candidate, but his presence would not be quite as interesting, beause we have no real connection to the character since have only heard stories about him. Orochimaru, on the other hand, is known by both Kabuto and Madara alike, and is not lacking in power whatsoever. His presence would also be the most shocking, since Kabuto has already absorbed the remains of Orochimaru. Whoever appears will be interesting, I'm sure, but I really want to see this be the case!