If we wanted a single arc thoughout the show, get rid of Ravaa and Satan, start with Unalaq as the sole antagonist of Book 1, do not sever the past lives, have him when defeated say something about the Red Lotus. Korra still opens the portals up and airbenders start popping up, with Korra still not having mastered airbending. Come Book 2, we get Zaheer and his crew, I would be fine with just Book 3 as we got it. Insert timeskip.
Then new Book 3 we get Kuvira reuniting the Earth Kingdom, but hopefully with some more development on her part. Then last season we get Amon with hopefully a better backstory, but this time adult Korra and Krew face him instead. Series ends, Korrasami is still a thing, everyone's happy.