whoever ends up being the final villain a way i could see them being defeated is being burnt to a crisp by a combination of Naruto and Sasuke. as they move in for the final blow i picture them running crossing each other. naruto puts his hand behind him and begins to form a rasenshiruken with sasuke putting fire in it then naruto throws it at the target engulfing him in flames.
i would've said amaterasu but it would probably burn naruto at the same time. nonetheless i see a technique like this happening between these two. not really sure what it would be called
oh yeah i also believe that Madara will be revived somehow
thanks for reading, enjoy your evening