So the way the story plays out, all of the main players seem to be Senjus or Uchihas. It seems to make perfect sense to me that Kabuto would be a Senju himself.

Think about his unnatural prowess with healing. He can even heal his own injuries nearly as fast as PTS Naruto. I don't believe Kabuto had already received his Sage Mode Training during PTS. It doesn't seem like he had many chances while under Orochimaru's thumb, and the flashback of him and training shows him cloaked and with the snake tail.

We've learned that the healing trait is common among Senju, specifically Tsunade and Hashirama. It also makes me wonder if Kishimoto had been planning Kabuto the entire time, even having him go face to face with Tsunade.

Kabuto has all but said he's trying to become the Sage of Six Paths himself. He is a step ahead of Madara in that unlike Madara, Kabuto has went through Sage Training. The only other Senju who has completed Sage Training is also the one closest to becoming the Sage himself, that being Naruto.

So he has the Sage Training, he has the Senju blood, what's he missing then? The very thing he is most interested in, a Uchiha, more specifically, Sasuke.

Yes, Orochimaru and Kabuto both had very similar goals. What is it that Suigetsu found in Orochimaru's hideout, that he wants to show Sasuke? More than likely, it's the secrets to unlocking the Sage's power, possibly secrets passed down from the Sage himself. Orochimaru always said he wanted to unlock all jutsu, but why? Not just to know all jutsu, but to be the Sage of Six Paths himself, the creator of all jutsu.

Only Orochimaru, much like Jiraiya, was unable to completely master his Sage Training. It follows suit, that both their students would go on to master it themselves.

We look at Kabuto's past, and we know that a Leaf village medical Jonin found Kabuto as a baby, the sole survivor of a battle at kikyo Pass. He was raised by this Medical Shinobi, and later on for some reason, was recruited and trained by Sasori, and had a seal put on him used to spy on Orochimaru.

It's my belief that Kabuto's new guardian was an Akatsuki spy. He took notice of Kabuto's unnatural healing ability, even trained him some himself. Yet, kabuto as a child decided to just go to Sasori of his own free will? That makes no sense to me. The konoha medical jonin was obviously either a spy, or another person Sasori placed his seal on. Somewhere along the line, he took note of Kabuto's ability and sent him to Orochimaru, who at the time was a comrade of Sasori. Only Sasori also wanted to spy on Orochimaru, so he placed the seal on him, which Orochimaru later finds and releases.

Kabuto would naturally be angry at the world, knowing he had no family, his guardian was using him as a tool, and Sasori as well used him as a tool. He grew to revere Orochimaru, and learned everything Orochimaru could do. Orochimaru trusted Kabuto enough to keep him as his right-hand man, but he wasn't dense enough to believe that Kabuto wouldn't attempt to kill him if given a chance. But he could still use him, and his body when the time came.

See, Orochimaru kept Kabuto close BECAUSE he was a Senju. We all know that Orochimaru was as fascinated with the Senju as he was the Uchiha, being that he tried to clone Hashirama(And succeeded, despite the knowledge that he did). Orochimaru needed to keep Kabuto around for the day he needed a Senju's body, after he got the Uchihas. But he was too valuable to take over until then, he needed Kabuto's medical jutsu, after all.

But Kabuto took all of these plans into his own hands. He took all of Orochimaru's ideas, and built upon them... Edo Tensei, he perfected and improved it. Manda, he revived and improved him. Snake Mode, he mastered and improved it.

So that brings us to Kabuto's motivation: Taking Sasuke Uchiha.

It's the last piece, right? He's a Sage, he's Senju, and there is only one more surviving Uchiha, one that he desperately needs and will even side with Tobi for a chance to take him. Actually, wasn't that his only requirement for helping Tobi? Sasuke and one Zetsu?

I could go a step further and say that Kabuto is already a Senju/Uchiha hybrid of sorts. I find it hard to believe that in the 100s of years of Senju/Uchiha rivalry, not once did a pair of them break off and have a rogue affair... That child would of course be kept a huge secret. Maybe that child was Kabuto's real parent? Dammit, this would TOTALLY explain the Red eyes....

But that seems crazy and is a completely farfetch'd assumption. However, Kabuto is the only non-Uchiha to have AB type blood. So whatever.

You just wait. Tobi's gonna eventually come out and be like "IIIIII was the one who put you in Kikyo Pass as a child!"

Weird... I thought that Zetsu was a hybrid due to his Senju DNA and recording abilities like Sasuke used to show Kabs how he killed Orochimaru.