I think that Orochimaru may be Madara's Trump Card or have something to do with it. Orochimaru was brought back yet has been doing nothing but playing Sasuke's lackey, but I don't think he would have been revived if he was going to do nothing but play servant.
Reasons:
1. Kishi brought Orochimaru back for a reason and if Oro is somehow connected to Madara that would explain it.
2. Orochimaru is older then Obito so the time line works with Oro having met Madara, I think its possible that Madara corrupted Orochimaru.
3. Maybe the plan was for Oro to work against Obito, so that Madara would have an ally against Obito if he ever betrayed him.
4. It was never explained how Orochimaru got his hands on Madara's DNA or corpse, but he got it somehow or Madara wouldn't be back.
5. This would also explain why Oro became so interested in Senju DNA and the Sharingan.
Reasons:
1. Kishi brought Orochimaru back for a reason and if Oro is somehow connected to Madara that would explain it.
2. Orochimaru is older then Obito so the time line works with Oro having met Madara, I think its possible that Madara corrupted Orochimaru.
3. Maybe the plan was for Oro to work against Obito, so that Madara would have an ally against Obito if he ever betrayed him.
4. It was never explained how Orochimaru got his hands on Madara's DNA or corpse, but he got it somehow or Madara wouldn't be back.
5. This would also explain why Oro became so interested in Senju DNA and the Sharingan.