Im not usually one to post theory's, but heres what i personally think, i dont want a debate its just my input to whats going on.
As we all know, Kishi likes his surprises, so im not ruleing out anything.
First off, i really believe that Tobi is NOT OBITO, things dont work out timeline wise and it would be a stupid move for Kishi to make.
I do however think the likely candidate is Izuna, just a few bullet points and my guesses why:
-Timeline matches:
(the first obvious thing), born in the founding Eras, which is why Tobi has the knowledge.
-Death:
Izunas death doesnt have that much information about it, it says he loved his brother which is why he gave his eyes (i wouldnt be surprised if Madara could Kumui), according to Tobi (which we also know lies about some things) which is 2 things, they have a similar goal, and also family love.
unless Madara did take them by force, which gives Izuna a reason to hate him thus Tobis dislike.
-Family Links - Kumui:
Izuna is blood tied to Obito, this makes him a perfect vessel for Obitos eye and mangekyo ability, less chakra drain as to being an uchiha, and as far as i can guess, Izuna probably had Kumui, thus Tobi being able to use it as effortlessly as he does.
-Bijuu:
Madara could control the Kyuubi, this gives Izuna a good chance to be able to do the same.
-Random - Zetsu: we know zetsu is part of the earth and part of Hasiramas cells, (bare in mind Hasirama could heal)
Tobi also has Zetsu like abilitys in the sense he can reform.
My theory on this is:
Depending on whether Madara Stole the eyes or Izuna gave them to him is what we dont know for sure, but eaither way the outcome is similar, if he took them, gives Izuna a reason to hate Madara, if the eyes were given out of love, then this gives madara a reason to ressurect him.
From Death Two outcomes:
(Izuna was revived one of 4 ways, Zetsu with hasiramas cell healing, some unknowen jutu or by Madaras Rinnegan when he got it, or simply and trollably by faking his death)
Love:
Madara and Izuna were close brothers, which means they have similar goals ( they could have had a promise to each other they would revive one another this eventually leads on to Nagatos Rinnegan with the life giving path).
Madara couldnt take the pain of loosing his brother, he seeked someone or something to help resurrect him, thus Zetsu's purpose (we know Zetsu is old i mean the main Zetsu not the 1000s of little ones we seen in the war or an amplifyed version as he was just created).
The other way is that when Madara acquired Rinnegan he revived Izuna.
Eaither way Madara also benefits as he has the chance to live again, hence pains original purpose with his rinnegan.
Izuna lives and finds the world declining through the century's, seeing all sorts of struggle, death and pain from so much wars and fighting, turning him into the **** that tobi is.
Hate:
Madara killed his brother (or injured badly to the point of death) in order to get his eyes, this leaves Izuna with a mighty grudge as he feels his brother betrayed him whom he loves dearly (could he have killed madara while hes unknowing? hence madaras final death), someway or another he regains strength and seeks revenge aswell as seeing all the hate and hopelessness during times of war, thus turning him into the **** that tobi is.
Eaither way, Izuna got so screwed up from whatever hes been through he wanted to use the infinite tsukioymi to end it all and turned into one nasty bastard.
Thus he followed his family through the Eras and kept an eye on Obito, hence why he was there the day Obito died to take Obitos eye which he is fully compatible with, he kidnapped Rin and forced her to implant the eye.
As for the mask:
Facial disfigurement: Izuna got injured and/or he couldnt completely blend with zetsu.
The obvious reason is Not wanting to be known/keeping on the low down, through the ages people knew of his story, so he hid the best as possible, or other great ninjas of the time could put a stop to his plans if they found out.
^ This is just a quick overview of what i think, i haven't slept so please don't flame.
Cant wait till next chapter ^_^