I have an interesting idea/theory. While its not the most fleshed out, I think its at the least plausible.
Nawaki was the grandson of the first Hokage, a direct line to the Senju bloodline. According the manga, he was killed when an explosive trap set off and his corpse was so mangled that he was unrecognizable, Orochimaru at the time was with him...
As far as I know, wanted to use the First Hokage's cells to create a decent clone of him(Yamato), he also empowered Danzo with the first Hokage's cells to enable him to use wood element jutsu and sharigan for extra power... But did he really use Hashirama's cells?
The basic idea is that, because Hashirama corpse would have been to far decayed to extract anything useful, Orochimaru would have had to use a direct decendent, Nawaki, someone who had the potential to achieve what Hashirama was. Orochimaru was supposedly Nawaki's sensei, at least according to the wiki as he was there during his death. His corpse was so mangled from his death that it was unrecognizable. Is it not past Orochimaru to hand over a corpse that wasnt who he said it was?
While the idea need to be more fleshed out and fitted into or out of the realm of plausible/crazy, I think it at least has legs, I want to see what everyone else thinks.
Destroy it, salvage it, comment and expand on it.