The mystery of Tobi actually reminds me of Final Fantasy Vii where Sephiroph has all these clones trying to become one, the difference here being that there's only one clone trying to become complete.
My theory is that Tobi is indeed a Zetsu clone, as they're reportedly as close to the real thing as clones can get and the fact that Tobi leeks white goo.
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Tobi alludes that he's only a former shell of himself, and while Zetsu is able to copy a person exactly, they in no way match the strength of the original.
Also, Tobi calling himself a "nobody" might be him alluding to the fact that he's a mere clone, and I believe his goal is to become whole (which I think may be mentioned in the manga, can not remember now).