It was heavily foreshadowed this tobi was a faker, Obito was the painfully obvious choice, but there were more interesting ones like Uchiha Kagami
True...after War Arc happened. Here's the thing. If we go back, it doesn't really add up. First of all, what's the point of only showing one eye, when he fought Konan and uses Izanagi? That wasn't needed. No one knows about that jutsu. Want to know why? Because it was added on since Kishi didn't plan it. That didn't make sense. The one eye (lol one eye) was made for Obito, but then retcon happened. He announced himself as Madara. If anything, it almost felt like it was going to be him but in Obito's body. Why Orochimaru got to be exclusive? He isn't. Not to mention why Zetsu was clueless on Tobi, all the sudden he now know him since Obito's recovery?
Now, why I said it should have been him? Well, if we go what if, you got Madara using Obito's body, leaving Kakashi wanting to end the misery for good, Naruto ending him because he's the one behind Kyuubi's incident, Sasuke ending him because he helped Itachi to massacre Uchiha, and Sakura ending him because he targeted Naruto and Konoha for destruction. They all connect. Now, if we go by manga wise, tell me, what business did Madara have with Naruto? Nothing. Then Kaguya just made it worse. That's why the yawn factor increased tenfold.
And that's why TG makes sense with all the top villains with Kaneki. It's all connected.