I think that there is a definite possibility that Danzo is Tobi. Especially after I found this japanese folk tale about a ninja named Kato Danzo.
Kato Danzo
Very little are known about the legendary ninja. It is said that Danzo was a master of illusion. For example, a historical document says that he swallowed a bull in front of more than 20 people. But another ninja was seeing it and said "never cheated, he is only riding on the bull's back," then people noticed that it was an illusion.
Today, researchers think his illusion as a type of group-hypnosis. In fact, ninja were studying psychology very well, so it was not strange that very skilled hypnotist were among them.
Historical documents say that Danzo wanted to serve to Uesugi Kenshin . But his art of illusion was so dreadful that Kenshin decided not to hire but to kill him.
Kenshin ordered him to intrude a house, and placed many guardsmen in and around the house. Danzo , however, noticed the deception, and throw his Shinobi Shouzoku (the black costume of ninja) tied with a rope over the hedge into the garden instead of himself. The guardsmen fired arrows to the dummy, then he pull back the rope. They believed that Danzo turned in the air to jump back to the hedge.
After that, he was called Tobi Kato , meaning "flying Kato ."
The story tells that a competent ninja cannot be always loved his master. He was, after all, too competent
After seeing this and knowing that Kishimoto uses actual folk tales to influence his writing this is a good indicator of possible things to come.