1. Kaguya stands as pure evil.
2. Madara serves as nothing more than a slightly more organized, cunning, and powerful Obito
-Like Obito, he had a dream that was just and good... but lost it because of events in his life (Izuna dies).
-Like Obito, he found a way to make a false reality (Except unlike Obito who found Madara... Madara found Kaguya and the Infinite Tsukiyomi plan on the Tablet)
When you really look at it, Madara's role is nothing more than Obito's role, the only difference was that we couldn't see who was pulling his strings until the Sage introduced Kaguya. We saw during Hashirama's flashback, Madara lost hope in the world. It is literally the same as Obito. The only reason we didn't instantly have a spam of theories about "New final villain." is because we didn't know of Kaguya and her relation to the Juubi.
While yes, Madara got all of this power, this serves to show just how much power Madara has. It is literally the same as when we went from Obito to Madara as the "final villain."
Obito: Kamui, Rinnegan, Controlled the Jinjuriki paths.... then we see Madara enter with Perfect Susanoo, and gradually show more and more power compared to Obito.
Now we see Madara display his power (Limbo, etc.), and now its being passed over to Kaguya (Who is manipulating the Zetsu).
Finally, and this one's just on me, Kaguya is a chick. Seriously, sounds sexist/reverse sexist... but seriously. Kishi has treated women like ABSOLUTE SHIT in this series. The strongest woman is between Tsunade and Mei.... and both are nothing compared to many male characters. Sure its a "sudden plot twist," and she'll hardly have the development any villain should (or any side character in Naruto, but hey that's another rant)... but at least Kishi made an attempt you know?
So....
1. She's representing a pure evil.
2. She's a chick.
3. She represents the origin of Uchiha AND Senju (thus the beginning of the Cycle of Hatred).
4. If she's the final villain of this arc, that means Naruto may not have fought, "The Man with powerful eyes," and thus suggests a final Naruto vs Sasuke.