if you ask me, if anyone should be able to control hashirama's power, it should be madara.. they're practically family lol danzou had no right and yamato simply got lucky
I personally think that attempting to control Hashirama's powers is precisely how Zetsu came into being. As we have seen Zetsu seems to share a number of similarities with Madara and Zetsu also seems to be plant like. Zetsu seems to have been with Madara for a long time and I think that is because Zetsu is a creation of Madara's. One chapter even descibed him as the magician's assistant, and seemingly posed the question of whether or not he is human.
I think that Zetsu is quite likely 2 separate people fused together and made one with both Hashirama's and Madara's cells. This experiement would explain how Madara would be able to fuse himself with Hashirama, the difference is that in Madara's case is that he is not two separate people made one but rather 2 separate bloodlines made one. I think it is also interesting that Zetsu seems to have unique eyes, after the fight between Sasuke and Itachi Madara asks him if he recorded the battle. I think that Zetsu's eyes also have special powers, perhaps a result of fusing Uchiha and Senju blood, this result could be an infant form of the Rinnegan Madara may have created by combining the bloodlines.
This also ties into the question of what is the byakugan? I have a theory that the byakugan is a natural progression of Uchiha breading with other clans, quite possibly Senju. The byakugan seems to possess some similarities to the rinnegan and also some similarities to the sharingan. It makes sense to me that sometime throughout history the Uchiha must have mingled outside their own people, as was almost undoubtedly true with the Senju. This combination could have given birth to new bloodlines. Just as the Uzumaki obviously possess qualities of the younger son of the Rikudou.
What I think Madara is trying to do, is combine the bloodlines in their purest form. That is why he needed Hashirama, Nagato, Sasuke, etc. Just combining any members of the clans is not enough to provide a perfect result. Interbreading between the Uchiha and Senju would not recreate the Rikudou bloodline because it has been diluted over time.
Zetsu is created by the Gedo Mazo tree. I doubt he has anything to do with Madara/Uchiha cells.