Madara's "trump card" is simply finding a way to transfer his soul into Obito's body so he can replace Obito's soul and use the body as if it were his own. This is indeed Madara's trump card. This why he still possibly had hope that he could be a jinchuuriki and didn't overreact when he lost his only chance. You know this could be true when he said Obito would get ahead of him before he tried to use this "trump card", and reject his soul transfer attempt. Now that Obito has no control, Madara has better chances.
Tell me what you guys think.