Very plausible indeed! Unlike some of the others, I think this is a good theory. Kishimoto is known for his surprises in the manga so this could be one of the greatest. And you do have a point there! Of all the people Tobi has encountered he was going to show his face to Sasuke and even helped him. The scars on his face could possibly be battle scars that he'll receive in an upcoming battle. Anyways, good theory!
Rikudo Sennin cannot be Tobi. He longed for peace and was the ultimate protagonist in the beginning. The sage defeated the ultimate evil. He's somewhat of a messiah. No way he'd be the man behind the mask.
Sages sons were mentioned in the manga to give us the background of the Uchiha and Senju and their curse. That is all.
Izuna had given his eyes to his brother and the proof is in Madara's EMS. And he fell in battle soon after so he is deceased. Izuna was used to explain the acquiring of an EMS and how Madara grew so powerful.
lol
