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I think at this point Madara's trump card can be anything. Previously I thought that his trump card was "forcing Obito to resurrect him". But if you think about it, so far Madara has been keeping a lot of his abilities in the shadows.
For instance, he has not shown us what his original MS abilities are. Does he has the Amateraru / Tsukiyomi combo? Or does he has space time jutsu or entirely unknown MS techniques? Madara commented about Sasuke's MS being the "straight swirl" type, so is his own MS completely different because the tomoe in his MS are circles?
And he was not using the full extent of his Rinnegan when he fought the current generation's Kages. Apparently he knew Yin-Yang release (since he created Zetsu with it), but he has not used it in all his "fights" up until the current chapter. Was he waiting for the right moment to unleash what he has?