Hopefully deliver.
Before Kishimoto does reveal who Tobi is, I would like to guess and claim that Tobi is Dark Naruto (conscious-wise). But he is using a body held up by Zestu's fragments.
If not that, then he is an illegitimate child of a Kage and an Uchiha. Possibly Tobirama as the father and a fodder Uchiha mother. To avoid conflicting between the two clans, Tobi was isolated and was only allowed "certain privileges." These privileges were not many, but it held his attention for the time being. One of those privileges was being introduced to a building that contain great texts. Those texts contained everything that has been revealed in the story thus far. Having read this, he begins to question his origins and why he is kept a secret. At this point Tobi mysterious learns of his origin, and realizing the reason he was locked away is because of this. Having learn this, Tobi begins to hate everything he is, and his sole justification is that it is world's fault for his hate. And so he begins his plan.
This is all just speculation and ultimately holds no fruit at all. But it is a possibility that Kishimoto could take.
I'm sure of it xd
