I have been reading many of the threads and frankly, isn't it obvious enough that Tobi is Kagami? Let's gather the information we know about him.
Firstly off, remember the agreement made between Tobi and Itachi to spare Konoha. Tobi agreed to help him kill the Uchiha instead, because apparently he wanted revenge on them for having betrayed him "decades ago". Decades, that makes him an old guy, don't you think? I don't think Shisui would have said anything of the sort, for example.
Yeah, he claims to have been betrayed by the Uchiha. Last I checked, none of the other possible identities for Tobi were betrayed by their clan, except Madara himself.
We've seen two of Tobirama's students (Danzo and Hiruzen), where is the third one gone? Uchiha's don't simply go missing like that.
Mei says Tobi thinks like Danzo. Of course, since they were both in the same team, both went through war, saw their master die, and none of them became Hokage. Hiruzen became the 3rd so he had other things to worry about than forcing peace on the world.
On the same topic, Tobi uses space-time jutsu just like his master, Tobirama. He surpassed his master, like every other apprentice in the series does. Which is why he could read Minato's moves later on. The "Tobi" is a reference to his master's name.
Kisame knew who he was. And Kisame was a veteran rogue ninja, a bit like Orochimaru in some way. He got around by the high ranked leaders and veterans. Just like Orochimaru knew Danzo very well, it wouldn't surprise me less that Kisame also knew Kagami's true identity.
Not the best argument but worth mentioning, they have the same haircut & physical aspect (both seem to be pretty tall).
And finally, who else could he be otherwise? By now every other solution seems gone.
-He can't be Shisui because their sharingan have different abilities, and both of Shisui's sharingan have been found/destroyed already.
-He can't be Madara because as we've all seen, Madara is dead.
-He can't be Obito for many, many reasons.
-All the other Uchiha have been killed anyway, while Kagami hasn't been seen not even during the Kyuubi attack...
I had other things in mind but not coming back right now. You'll see, he is Kagami