He even seems to know jutsu that never existed until after he died...so what, now he can see into the future? He has become a god now. It would have been better if Tobi was Madara and managed to combine himself with himself to be complete. Literally become alive again...a mortal. A powerful mortal, but mortal none the less. At least then he wouldn't be so hax as he is now. We face another problem on how he is going to be defeated. Who or what can defeat the now immortal over-powered Madara Uchiha that seems to know every jutsu and holds all the keys? I have never once seen a villain like him in any other media where they just have no weakness. And if he does have a weakness, I can't think of what it is.
The Kages? They've been trying to do it and seem to keep failing. I am can't think of anything they can do to defeat him.
Possibilities:
Sasuke? He could use the Izanami and force Madara to leave. Cop-out? I think we passed that line of "ridiculous convenient cop-outs" when Madara summoned himself using the Edo Tensei. He would also be seen as a hero if he does it and once again diminish Naruto's role as the main character
Naruto? Well, if he can defeat Madara, who seems to be more powerful than Tobi and Sasuke combined, then what's to stop him from obliterating them? And what about Tobi? We haven't begun to look into him and yet while he was menacing as the leader of Akatsuki, he is now seen as a chump. I don't know anymore.
The fact that Madara is immortal and unlimited chakra is what makes it such a drag. He has no weakness and he has infinite stamina. What can beat that?
Tsunade and Dan. Look fellas, he was her first love an Tsunade also loved Jiraiya too. As people said about Sakura, just because he was your first love does not mean it is your only love. I think we can say indefinitely that Tsunade loved both, but still has issues to resolve around Dan and she got her closure. I don't see her just ignoring Jiraiya because she was just as sad when he left to die. And no, Sasuke is nothing like Dan.
Well, if you ask me, the harder the villain to beat, the better it is. After all, Naruto's the hero, right?? in order to have a final villain worthy of the series, the hero's chances would have to be heavily stacked against him. That's why I think he would be the final villain. Most of all, Madara represents EVERYTHING Naruto stands against, even more so than Sasuke.
But Kishi has stated time and again Naruto vs Sasuke will be the final battle. So we don't know what will happen from here on out.
As for the bolded, if you go back to the chapter he was revived, he did mention Nagato. That would mean he lived through the middle of Hiruzen's reign. So, obviously he'd have some info on ET and began investigating it.