Madara's Trump Card
As we all know Madara said he had to use his final trump card before Obito anticipates him.
He said it in this sentence 'Obito became like the Sage of the six paths, he might anticipate me before I can use my trump card'
This means Obito is able to defeat him if Madara doesn't get the chance to use his trump card. although this seems obvious keep in mind that this are Madara's thoughts so he is confessing in a way.
Madara is in a hurry about using his trump card as he immediately takes action but for some reason he doesn't perform any kinjutsu or ninjutsu. Instead he starts fighting Hashirama!
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This makes me think that Madara's trump card needs preparation or Hashirama in some sort of way.
In the same way of thinking we can discard the fact that he needs preparation, this because he stopped Hashirama and engaged in battle while he could've kept a low profile for his preparation.
He needs Hashirama's abilities he is not trying to safe Obito from being defeated.
Now is the question: What can Hashirama mean to Madara aside from being a rival, in what way could he contribute?
We can consider it safe to say that we know 99% of the Rinnegan techniques. We know very little about the Sage's body tho.
I leave it with this, Madara needs Hashirama and next time we see them on panel we will get some information about the events after the Valley of the end fight. And we may as well get to see why Madara needs Hashirama.
Feel free to speculate and lets come up with some theories about this, my guess goes to the creation of all things since it's unknown what it can achieve in wrong hands, what we do know is it can be considered an infinite tsukiyomi as it is able to create everything out of Yin and Yang chakra, perhaps this is what Madara has been plotting since the start instead of Infinite Tsukiyomi.