I think it is obito's father, it would fully explain the resemblance, the timeline, the awakening of MS, the distance between him and minato (the way he spoke to minato was odd considering their history), the world renowned status amongst missing nin (kisame), the resentment towards the leaf, having huge ambitions of saving others (as obito did, he wanted to be hokage and protect his allies)
Also it is unlikely that minato would not have immediately retrieved obito's body from the cave-in, after dismantling the members of the ambush ofcourse (as sharingan bodies can not be left lying around due to their valuable dojutsu), he probably the retrieved obito's body and gave it to his father at the bridge to take back to konoha for a proper burial.
I would also like to point out the parallels between Naruto and Tobi, Naruto seems to be a yang powerhouse where as Tobi is a yin power house, also To i's son (assuming the theory is right) had the same dreams as Naruto growing up, and behaved in a similar way, thy also both had great potential.
Also adding to the yin yang aspect, Naruto lost his parents where as obito's parents lost their child, Naruto parents bestowed their power onto him in death, whereas it was through he death of Tobi's son that he gained his power.
Another reason I find I hard to believe that Tobi is obito is from the fact that he was able to enslave Kurama, you need power chakra to genjutsu Kurama, a child like obito did not display such things, even Madara had to wait until he was prime before he could enslave Kurama.
Not only was he able to enslave Kurama but he also summoned him, that takes massive amounts of chakra due to kurama's size, I think this person was obito's father. Infact I have even more on this.
