Trump card and the fake plan?
Check out again what Madara thought....
He does not want Obito to anticipate whatever he is about to do. Madara got aware because Obito is close to SO6P. So there is something that SO6P is capable of learning about opponents moves before implementation.
Can you guess what the trump card can be?
What is still a secret?
What is still not shown?
The answers are simple and straight forward.
Its Madara's EMS technique(s)...the Trump card OR there's a second one too we'll discuss it soon.
What is still a secret? Remember Madara's death?
The truth we all know is that Madara obtained Hashi's DNA during the fight and faked his death.
But how Hashirama ended Madara was totally different. There was not a single opening for Madara to obtain Hashirama's DNA and not even a fatal wound on Hashirama's body.
Madara's two main things to be accomplished was
1] To obtain Hashirama's DNA and fake his own death and erase himself from the History.
2] To become Juubi's Jinchuuriki and implement Infinite Tsukuyomi.
Obtaining Hashirama's DNA and no further battling with him was the turning point for Madara. So Madara's one and only option was to use his EMS technique(s). There has to be a condition for using his EMS technique(s) because even after several battles with Hashirama he lost all of them but won the last by faking his death. Also he had not used it in the war still even when his whole dream is fading away in front of his eyes.
In the past, it was Hashirama who was stronger than Madara and that is why Madara might have used his EMS technique on Hashirama to win over what he desired. The similar condition is there now, Obito, the Jinchuuriki, is stronger than Madara and to win over his own mortality and the Juubi, Madara will use the EMS technique(s) on Obito such that Obito will be unaware of it. And only other rinnegan user can see through it. So there exist a condition to use EMS.
So to become the Juubi's Jinchuuriki, Madara will once again use the EMS technique(s).
And about Infinite Tsukuymi plan, this plan is the biggest fake dream that Obito is living under.
We all know Madara transplanted his rinnegan into Nagato with a hope to be revived in the future. But what were the chances that Nagato will survive till then and what were the chances that Nagato will revive Madara? So Madara needed someone who can actually move his plan till end by guiding Nagato. Not any one but someone who'll be benefited by that plan. Soon Madara got Obito, went through his past and made a plan of Infinite Tsukuyomi. How he achieved it?
Through Rin. Why during that time Rin was kidnapped? Why a scene was created in order to protect Konoha? Why the Bloody Mist village? It was all planned. Madara was aware of Konoha's Will of Fire thing that every Konoha shinobi follows. He knew, at some point, Rin will sacrifice herself to protect Konoha and that too by the hands of Kakashi. Rin's death affected Obito the most and also the shinobi world outside the Konoha for example : Bloody Mist Village, made him think about Shinobi system.
Madara nailed these two things in Obito's head and drive his attention completely towards the completion of resurrection of Juubi by telling him about a dream world created by Infinite Tsukuyomi through a Genjutsu which itself is an illusion.
And now Obito is one step away from the actual plan. He will do whatever he can to accomplish it. But what if he finds that there is no such thing as Infinite Tsukuyomi? It will destroy his life for what he spent everything. He will try to kill/seal Madara permanently. But to avoid this, Madara will use his EMS technique(s), take over Obito's body and become the Juubi's Jinchuuriki
Everything comes to halt at Madara's EMS technique(s) which is still unknown.
The other possible Trump card could be his brother Izuna Uchiha.
Like every past characters revived back, Izuna is the other one. Also Izuna's death is also a secret. No one will believe someone powerful like Uchiha Izuna's death by a mere sword attack. Also there was a huge time gap between the two incidents. One when Izuna was attacked and the other when Madara returned with EMS. How can Madara let his only brother die like that? Without any revenge?
The second Trump card is just a thought. But the first one is the most possible one that I think.