Ok. I know this is a half baked idea that has no real ground but just hear me out and rib me on it later cause the idea amuses me anyway.
We all wonder of the mystery of Kabuto so i got to thinking about his past and who his parents might be.
Looking back upon Dan and Tsunade. The obvious fact of them being highly skilled medics and the fact Kabuto and Dan looks very much alike was an obvious start.
Then considering it was actually Orochimaru who placed Kabuto at the battle of kikyo pass.
I wondered what about a 5 year old would interest him so much as to place him in such a way even before Sasori got to him.
The time Kabuto was born and the fact Tsunade i think was a couple years older then Dan who died at 27 leads me to think she could have gotten pregnant and Oro steals the baby.
Yeah crazy stupid idea and such. but he wants the abilities of the first and tried Nawaki thus killing him by accident and his experiments failed so he instead tries with Tsunade's son.
So i look at abilities. Hashirama could turn chakra into a source of life and make plants while Kabuto can use chakra to create new cells and basicly create new life within himself thus his incredible healing. and the old blood he holds. seems rather beast like. much like Hashirama himself might have been without his chakra crystal helping him control his inner power and thus help control the bijuu.
Yeah that's about it. A stupid theory with far too many loose ends maybe but, i like it and maybe one of you will too so discuss and be merry. ^^