Re: Hashirama Senju's death
On a more serious tone... Personally, I think that wood release is the result of a forbidden jutsu created by Hashirama with the two-fold purpose of:
a) Turning him into a mass of cancer (infectious, btw, as lots of people seem to have "contracted" Hashirama) that has cell reproduction that's both invasive and out of control... Which allows him to overcome the limits on mitotic regeneration that having cells with effective Hayflick limits results in. Dude can sit there all day regenerating, he's effectively immortal.
b) Turning him into a plantlike being (demonstrated by Danzou having his arm turn into a tree), probably in order to still maintain tissue differentiation, with the added plus of acquring Wood Release.
The thing most people miss is that Konoha-age Senju seem to be quite proficient on human experimentation, as seem with Tobirama creating Impure World Ressurection... Hashirama probably also had in him, but chose to experiment upon himself instead.
Personally, I don't think Hashirama died at all, especially given the power level he had at his command, as Madara has demonstrated... He either faked his death, and is still around (due to being, basically, a cancer mass), or somehow lost control of his plant jutsu, and turned into a tree.
Either way, I don't think that dude died at all. I think we may still get to see him, alive and kicking, I might add.