He's wearing a helmet. Probably an adamantium helmet.
Obito would have been a teenager during that time period. There was nothing to suggest an age of Tobi during that fight, so it being Obito does not put a hole in the plot.It can't be Rin, as the frequency of Kamui's use by Tobi suggests only a natural Uchiha body could withstand it. I'm still not giving up on it being Izuna; he could be just trolling Kakashi to win the fight. Besides if it does turn out to be Obito it would plot hole the timelines vis a vis fight with Minato etc.
It wouldn't be the first time Kishi ignored previously stated "facts" about the sharingan.Also, the amount of times he has used Kamui means he should have gone blind by now if all he has was normal MS, like Itachi, Kakashi, pre-transplant Sasuke/Madara. The fact he can still use his eye must mean it is an EMS, which follows that it isn't Tobi's natural eye but rather a transplant.
Kurama also said that Sasuke's chakra was just like Madara's, so maybe all Uchiha has similar chakra. Or maybe he was referring to Tobi when he said that, not knowing who Tobi really is and only that Tobi called himself Madara.Thirdly, the fact that Kurama thinks that Tobi is Madara must mean either Tobi posseses Madara's chakra or his own is almost indistinguishable from Madara's. The former could mean it is Obito through some trick, whilst the latter could point to Izuna.
Plus, why would Izuna wear a mask to hide his identity? It would be rather anti-climatic to hide his identity for this long, only to reveal him and have nobody recognize him or know who he is.
Honestly, if you're not convinced by this point that it is Obito, then it's going to take seeing his face and having Kakashi say "...O..bi...to...!?" Because Kishi has done everything possible to let us know who he is, except for show his face at this point.