Just thought of something plausible, which can possibly tie together about who tobi actually is.
Obito- the theory itself isn't bad, but doesn't tie anything together, the time frame from the incident with the mizukage, and minato's battle with Tobi would suggest otherwise. Height, strength, time frame, and techniques suggest that it's impossible for Obito to be Tobi.
Kagami- Doubtful seeing he was in the second hokage's squad, close to the leaders, close with danzo and the third hokage, I think he was more patriotic towards the village rather than his own clan.(just an idea, can't really confirm ANYTHING on him, due to the fact we know such a limited amount on him and his background, but nothing suggests he could possibly be tobi)
Now, going back, tobi apparently lost his light in his eye when he was fighting konan, which means that something is seriously wrong here...He stated he had both Uchiha and Senju DNA both, what if he was actually and he had EMS, does this mean that EMS can't spam the Uchiha Forbidden techniques unless somehow Tobi did not actually have the EMS, hell I didn't even see MS being lit up there... hell I don't think I've ever seen this baws go EMS, so how can we know that he is even an EMS user at all... He is old, and that's obvious, how old? We can't really tell, he might be the first uciha, and a Uciha Overall...
BUT what if he is not an uchiha, but one of yamato's clones, or someone from the Senju clan... he has senju DNA in him, has zetsu-like abilities.... he uses some sort of space-time ninjutsu, so what if he is indirectly one of Orochimaru's subjects, or a senju related to the first hokage, and has regenerative abilities similar to his... Because everyone is figuring him out to be an Uchiha, but none of the know Uchiha have be known to have similar abilities and ambitions to his except madara himself..
Possibilties
1. Tobi Is an Edo summoned by that was actually put un Hashirama Senju's body, but had somehow implanted Uchiha eyes in himself, and released from the Jutsu's grasp and had researched his own body to behave differently than normal Edo's, no attempting to not be injured at all, but attempting to be invulnerable, except when attempting to land an attack.
2.Tobi is a Senju/ Clone of Hashirama (which in my opinion could possibly be a plausible idea), but without background knowledge, it's a scare idea at the same time.
3. Tobi is an Uchiha (kagami, Obito whoever le fuq)
4. Tobi Is Actually Madara's Clone, thus shares the same interests/memories with him (which is also one of the more probable ones)
Ideas might be a tad far fetched, but seeing Tobi's Inability to perform an MS, or any type of MS jutsu seems rather appalling, as I doubt he has knowledge or rather, lack the ability to perform the jutsu because of the state that his body is in is incapable of performing Susanoo, casting MS, or Tsukoyomi, and other jutsu's, BUT, this could also imply that he is Madara, but had MAYBE somehow, gained the ability to split(similar to Muu's Jutsu) and lost half his body in the Madara/Hashirama Fight, whilst the other half had survived, but without the other half of his body, he is incapable of neither dying or Fighting to full ability...
Doubt we'll know the truth about this mutha****a for a while, Kishi is using more Plot-no-Jutsu to keep us hanging, but there are a TON of possibilities, and I doubt we'll know the truth for a while, until tobi gets ****ed up... or when Orochi/Sasuke are talking we might get a revelation on who the **** Tobi is...but you must not forget that Tobi could be a Senju, just as much as he could be an Uchiha...because he lacks MS and has stated that he has gained both Uchiha and Senju DNA to be capable of using the forbidden technique of Izanagi/which hint hint, Itachi somehow was able to use as well...what if mikoto Uchiha was a Senju? O.O, we still have to talk about how Itachi was able to use Izanami...WHICH could also possibly mean he and itachi are somehow related.../ has Abilities similar to Zetsu's which are derived from the First Hokage... and is one tough son of a bitch...