I still have a huge theory that Orochimaru anticipated all of this and now at the end of the fight with both sides pretty much whittled down, this is his perfect chance to strike, just as soon as Madara or Obito get taken out. I don't know how he'll go about it just yet but there are so many opportunities. Snake sage mode might enable him to be able to control all of the Hokage, Yamato might come into it, Orochimaru may use that Hashirama poison Kabuto crafted (seriously, that stuff is a little bit too convenient to just introduce then leave yknow?) or maybe he just has a total huge plan or will strike Hashirama when his back is turned against Madara in a pivotal moment.
All I know is that I have 100 reasons why I don't trust Orochimaru. He's planning something, I promise you.