Don't read if you don't read the mangaaaaaaaaaa
I'm agreeing with the "Madara isn't actually Madara and the real Madara was in the coffin" theory. If the sage was in there, surely Madara could just use him for his plan? Wouldn't make sense if Kabuto let him take over everyone in the world, including Kabuto, though, so it could make sense it's the sage. Otherwise, Madara's brother is a good call, don't think there is anyone else we know of that could be in there. Maybe one of the sage's sons, I don't think they were immensely powerful though. And surely he'd have got them both.
Regarding the new six paths, we still don't know how Nagato got the Rinnegan into his paths, so it doesn't really make any sense. Tbh they probably should've shown Madara making them so they could've explained it. The eyes just appearing in them is lame. Where did they come from and how did they get there?