I have done this a few times, but I'd like to spread my thoughts
I think
Tobi = Kagami Uchiha ..why?
1.Old Face, about same age as Danzou
2.Was in a team with Danzou during the Ninja War
--> explains why he wasn't surprised by any of Danzou's techniques
3.The team they where in was Tobirama's team..
TOBIrama
--> Tobirama was the first Shinobi with space time Jutsu
--> Explains why Tobi could use S/T-jutsu without MS
4.The only two of Tobirama's former team not present at Demon Fox's attack --> D & T
--> see
You must be registered for see links
and than look at
You must be registered for see links
to see that they are clearly missing
5.The only two characters besides Yamato with Hashirama's cells --> Danzou & Tobi
6.The only two characters with a massive collection of Sharingan --> D & T
7.Both contributed to the Uchiha Massacre
8.Danzou never acted surprised about Tobi either, as if he knew who he was
They probably where close friends before, with the Uchiha massacre as a part of a plan and vision they shared. But it seems along the way, Danzou had more interest in becoming Hokage and Tobi (Kagami!) in bringing the whole world under his control.
I think while they didn't agree, they decided to just split up without a fight, because they where comrads once.