This theory is a stretch, but it is worth addressing. Currently, I would not say that Naruto is related to Hashirama because of Mito. Rather, I would say that Naruto's relationship to Hashirama is due to Tobirama, who is Hashirama's brother.
My opinion here is that Naruto is a descendant of Tobirama through Minato. While I don't know exactly this, there has to be a reason for Minato being a little Tobirama from childhood. Besides, I think Minato feats perfectly the role of Tobirama's grandchild, on whose head Hashirama wanted to swear when dealing with Madara's case. See how things are about Minato and Tobirama.
(1) Minato seems to be under the tutelage of Hiruzen, even before Minato became Hokage. This make Hiruzen someone who look after the orphan grandchild of his Sensei, Tobirama.
(2) Even as child, Minato's Shunshin was beast enough to best some Kumo jounin. This ties him with Tobirama who was said to be the fastest shinobi in his era.
(3) Minato can be said to have inherited Tobirama's signature Jutsus - the FTG. This jutsus is efficient only with their chakra, and actually seems to work just like a KKG. Without Minato's chakra, you need three Jounins trained by him to use the jutsus. It just show how the Jutsus is tied to the blood of the family - in this case Tobirama's.
(4) Minato was genius enough to best Orochimaru on Hokage proposal candidate. Note that this happened even before Orochimaru's incident was discovered. Minato inherited the will of fire on a scale larger than Orochimaru's along with his knowledge of Jutsus. Just notice that Orochimaru was already a Sannin.
etc.
I think there are already many hints that tie Minato to Tobirama, along with Sensing techniques. Actually, in the Obito's chronicle, the teamworks there was more like a family business - Naruto+Tobirama, Naruto+Minato, Minato+Tobirama, etc. The teamwork was naturally tied to Tobirama and his Jutsus - that is FTG and Kage Bushin.
