This is not an Obito Theory ( Even though I think Tobi is Obito)
This is a why Tobi isn't Izuna
1. No emotional impact, Izuna has no connection with any of the good guys, they probably wouldn't even recognize him, that makes the whole wearing the mask thing pointless. Remember he is wearing the mask to hide his identity from the characters not just us. That means that he is a character that Kakashi and Might Guy already know, or as I said before the mask is meaningless.
2. If he is Izuna then he drops from main villian to Madara's little brother. He would no longer be a stand out character.
3. This guy's connection to Madara has yet to be covered, his reaction when he saw the Edo Madara in the coffin scared him, this is not the reaction of a little brother seeing his big brother resurrected. Tobi doesn't want Madara back, he wants to rule by himself.
4. This guy is bitter. Not just with the recent scenes, the signs go back to the Five Kage meeting, he talks about hope and how meaningless it is, in the Tobi vs. Konan fight, he states that Naruto is just a naive kid with a immature dream. Why would Izuna be that bitter?