I guess Kishi made a hint, when he showed us the dead body of Izuna in the past.
We all have seen his dead body in the coffin so everyone is assuming that he's dead.
But one thing which bothers me really much, and what I call a hint, is , that Kabuto was not reviving Izuna as an ET.
This here kinda seems really strange :
The person, who seemed to be Tobi, was resurrected by Kabuto - Madara Uchiha.
The person, who seemed to be dead, but was very easy detectable, because everyone was knowing where he was settled down, wasn't revived by Kabuto - Izuna Uchiha.
I'm 100% sure, that Kabuto was aware of how strong Izuna was and he knew, that Madara and Izuna were equal on par. I also believe, that he was looking for Izuna, instead he found Madara.
This, without the other facts, outclasses every specticism to me, that there is no way he could be Tobi.
Either Izuna is Tobi or something else happened after his death, so Kabuto could not find him.
But to me, it quiet sounds logical, that Izuna could be Tobi, especially, when Kabuto did not revive Izuna, because he couldn't find him.