In my opinion, that would make any spin-off/gaiden/continuation of the story a lot more interesting. Specially if that third power has a villainous character. Since Kaguya is basically the mother of all shinobi, there must be another power that could rival the abilities of an Uchiha and a Senju. Toneri had some spark about that, but it didn't really pan out. I'm putting my bets on Mitsuki.