tobi is izuna (final theory)
3. Tobi is a Zetsu/Madara clone
-It would be a very underwhelming revelation if Tobi was simply a clone, so much so that there shouldn't even be a huge hype about it
-Tobi likely predates Zetsu. Conversations between Zetsu and Tobi has implied that Tobi was (one of) the original conspirators of the plan, while Zetsu is not
-Madara likely wouldn't really take Kabuto's suggestion that Tobi may not follow along with the original plan if Tobi was just a split copy of himself
-If Tobi was a Zetsu clone of Madara using Zetsu's powers, why did he age? Why not have multiple copies of "Tobi"?
-If Tobi was a Zetsu/Madara clone, why wear a mask at all?
4. Tobi is the Sage of the Six Paths
-With the revelation of Madara's Rinnegan and the fact that Madara knows who Nagato is, the theory of Tobi being the Sot6P is much more unlikely as it implies that Madara gave his Rinnegan to Nagato. If Tobi should be the Sage, then he probably would not need Nagato's eyes, and would likely resurrect Madara himself
-The motive for the Sot6P to undo everything that he's accomplished is very questionable
Izuna Theory's Flaws:
1. Tobi does not ever turn off his Sharingan, something seen with Kakashi and Danzo, who are both non-Uchiha users.
-I think this is the biggest issue that this theory faces. However, you could observe Itachi and he rarely, if ever turns off his Sharingan. The fact that Tobi does not turn off his Sharingan could in fact, be a plus for the theory. It is known that non-Uchihas spend much more Chakra to keep the Sharingan active than Uchihas, ergo why Kakashi and Danzo keeps their hidden/blocked. However, the Fact that Tobi has both his eyes as Sharingans and does not keep one blocked may mean that his Sharingan does not drain his Chakra as quickly and that he is an Uchiah
-We've seen Tobi in situations where he is not in direct lieu of combat and we do not see the state of his eye. It is possible that he does turn his Sharingan off sometimes
-We do not really know if it's just Non-Uchihas who cannot turn off their Sharingan, or if the rule applies to anyone with transplanted Sharingans. The only transplants we've seen so far are those of Kakashi and Danzo, who are both non-Uchihas. It is possible that any transplanted Sharingans cannot be turned off if it doesn't belong to that person, Uchiha or not.
2. Izuna is said to be dead according to Itachi and Tobi.
-Itachi was misinformed about the identity of Tobi and Tobi himself has lied
3. Izuna's motives for claiming to be Madara is poorly justified.
-Madara has accomplished much more than Izuna and it was explained by Tsunade herself in the recent chapters that this is likely due to the name striking fear and intimidation. Madara has achieved much more than Izuna ever had; unlocking the EMS; controlling the Nine-Tails; battling the First Hokage; etc
4. Izuna has not been seen using any "traditional Uchiha techniques.
-Remember, Susano'o is said to be rare. We have seen him use Tsukiyomi (Konan) or at least a powerful Genjutsu. He has yet to use Amaterasu
-His MS may not have Amaterasu/Susano'o. This has been seen with Shisui and Kakashi's MS.
Two Tobis:
We've seen both a Tobi with short and long hair. The Tobi with long hair was supposedly involved with the Uchiha massacre with Itachi and the Tobi with short hair was the one responsible for the Nine-Tails' attack on Konoha. It is possible that the Tobi with long hair was Madara himself, with the short haired-Tobi being Izuna. However, why Madara resorted to wearing a mask despite Itachi identifying him is odd. In addition, Itachi would have noticed the change in Chakra signature between the short and long-haired Tobis. It is likely that Madara died prior to the events of the Nine-Tails' attack and Tobi has always been Izuna.
Appearance:
Not much can be said here because appearances do not determine the character in its whole. Tobi is very, very old. However, he still has a youthful body . This could be explained by the Zetsu cells he uses. His face does match Izuna's face with thin eyebrows, a sharp jawline, and the wrinkles of an old man.
Motives:
More than anyone, including Madara himself, Izuna would hate Konoha. He sacrificed himself for the Uchiha but was betrayed when they sought peace with the Senju. This may explain his huge resentment towards betrayal; as shown when he attacked Kabuto for being an Orochimaru follower; his comments towards Konan and Nagato for betraying the Akatsuki and his threats towards Sasuke incase he was betrayed by him.
Another huge evidence that Tobi is Izuna is due to his behaviour over the course of the story. Most notably, his behaviour when he reminisced about Madara's life while first meeting Sasuke after Itachi's death. If you rewatch the episode, you will notice that he clenches his fist and talks bitterly when speaking of the founding of Konoha "What did my brother die for?". That is a lot of emotion that would be unnecessary acting from anyone else but Madara or Izuna.
Now, we've seen how ambitious Tobi can be. Tobi has shown a habit of taking very risky gambles to advance his plans, such as revealing Itachi's wishes and his own involvement in the Uchiha clan massacre to Sasuke, sending him to fight against the five Kage, involving himself with the equally dangerous Kabuto (although in this case Kabuto blackmailed Tobi into having no alternative) and even attempting to complete his Eye of the Moon Plan despite not having the full chakra of the Eight and Nine-Tails. With such ambition, Izuna may want to simply take the identity of Madara and replace him due to the dent in his original plan caused by both Nagato and Kabuto.
Assumptions:
-You need both the power of the Senju and Uchiha (EMS) to unlock the Rinnegan
-Izuna may be too weak to use the Rinne Tensei, or there may be a trick in using it which he cannot. Remember that Konan said “Nagato is the seventh path”. There is a reason why Nagato hasn't used Rinne Tensei to revive Madara for Tobi the entire time he was influenced by him. Also, there's a reason why Tobi himself did not use Rinne Tensei. As we've seen, he can clearly use the Rinnegan
-Izuna may have not died during his youth. He was able to simply transplant another Uchiha's eyes into himself. Given that Tobi is Izuna, he has already done so
-There is a reason why Madara died although he survived his fight with Hashirama. It may have something to do with the Stone Tablet (remember, he gained the Rinnegan shortly before his death. With the Rinnegan, he was able to read the entire tablet; so his death likely had something to do with the instructions on that very tablet
-Izuna's eyes = Madara's new eyes = Nagato's eyes; why Izuna claims that Nagato's Rinnegan was his in the first place
Theory:
1) In order to acquire the power of the Six Paths and be able to become the Jinchuuriki for the Ten Tails, you are required the Rinnegan, Sharingan and both the power of the Uchiha and Senju, and to have been able to experience Death, as the Stone Tablet of the Uchiha indicated.
2) It is possible that Izuna died before the founding of Konoha. However, he was brought back by Madara using the Rinne Tensei, killing the already weakened Madara (he chose to die to obtain the power of the Sot6P in the long run). Or, Madara was on the verge of death and had no choice but to bring the only trustworthy person he knows in on the plan – Izuna.
3) Madara knows of Nagato as he lived until Nagato's Era. With his death, he instructed Izuna to give his eyes to Nagato to activate it. Izuna is either not strong enough to activate it/use Rinne Tensei, or Madara wished Izuna to not have to sacrifice himself to resurrect him with Rinne Tensei, so they are using a third person to do so.
4) Due to the fact that Kabuto has used Edo Tensei on Madara, and that Nagato did not use his Rinne Tensei on Madara, Tobi has decided that he would not resurrect Madara as planned. Instead he would act as the Jinchuuriki of the Ten Tails himself and completely bypass Madara to complete the Eye of the Moon plan.