...by kaguya. I think after madara gets defeated, she's next. It makes logical sense, as she's meant to be even stronger than the sage of six paths, so it would be perfect if she's the last major enemy in the war.
Please no that would be lame af. That's like having a random final boss pop up in a fighting game after you finally defeat the overpowered, cheap, attack spamming main one lmao.