Hmm, one thing I noticed is that Madara mentioned that he "had" five brothers, however Izuna (the one who later "gave" him his eyes) is the only one we've heard about. At this point I'd assume that at least one died in war, to set up a parallel to Hashirama. But what happened to the other two brothers?
Well, let's remember that Madara and Izuna both had the Mangekyou Sharingan at some point. And how do you obtain the Mangekyou Sharingan? You kill the person closest to you. In Sasuke's case, he got it from killing Itachi. Did Madara and Izuna do the same?
Or in other words, will Madara/Izuna kill their other two brothers (maybe in some sort of deathmatch?) in order to upgrade their Sharingans? Seems like Oda might be setting that up as a way to show how brutal the Uchiha Clan was.
Plus, if that happens, it would make sense that Madara would later be so salty about allying with the Senju Clan, and then not even being Hokage. After all, it would basically mean three of his brothers died just for him to get the eternal Mangekyou, and after all that they're just like "Nevermind, we decided to squash this beef lol", making all his brothers' deaths for nothing.