[quote name='Graham Aker']Kabuto is awesome. But still a imitation just with upgrades.
The reason why I love him is for two things. Bringing back a zombie army(ITACHI!!) and Manda 2.0[/QUOTE]
Yeah,Kabuto is just an ''improved imitation''(and he doesn't seem to be much improved as well)
Orochimaru was at least confident while Kabuto seems to be sort of a coward.
now,to the topic:
well,there is a lot of evidence that Tobi is really Madara.After all,he obviously knows very much about Uchichas and Konoha's history. Not to mention that he is rather old(as can be seen by his wrinkled face) and has got enough chakra to summon the Kyubi.
Also Tobi's sharingan mastery is high enough to control not only fourth mizukage Yagura but even Kyubi.
Only Madara was known to be capable of both(summoning Kyubi and truly controlling it with his sharingan).
Therefore I think that Izuna might be in the 6th coffin since he is pretty much the only one who could pose some threat for Madara
(so far as we know of course).I am ignoring ''Rikudo Sensei'' theory here as his body should have decayed long ago...there may be some bones left but it still seems too improbable that anyone would find them...
or it could always be the other way around(Tobi being Izuna while Madara's body is in that coffin).