That's the way I'm seeing this. I was actually starting to see this as a possibility, cause as we've seen from Kakashi and Danzo, you don't need to be a Uchiha to have the Sharingan. And we haven't seen Hashirama's face on any part of Tobi's body as we have with Danzo and Madara, so whose to say that's the half of the puzzle he needed to acquire in order to use Izanagi? He could very well be a Senju or an Uzumaki who managed to find Madara's eyes. I'm conflicted cause I still find Izuna to be a possibility, but this is a close second for me, though I don't know which Senju/Uzumaki I think he would be.