I'll write the Hokage's regrets myself. This story has almost nothing to do with the original, so please read with an open mind [Hokage + Demonic Arts + Alternate Universe + Multiple Heroines + Divine Realm]
What are the Three Great Sage Lands plotting? How did Hiruzen Sarutobi become the Third Daimyo? What exactly is the Ōtsutsuki Clan? What is it all for?