I wish they could redo everything from the end of season 2 onwards but that's not going to happen and season 3's problem was the same of Korra's. They set out a plot for a full season and then had it cut in half last minute. Best to move on.
Assuming they can even do a 4th season, I would hope they return to the formula that worked in the first place and alternate between plot and filler episodes. Tot's neglected the latter and it caused S3 to be a boring long movie with no breaks for the characters to develop.
So maybe a S4 plot could be that the gods have left to become mortals like Iop did and the crew have to come down to the World of Twelve and actually journey again. See new things instead of crawling a dungeon tower for 13 episodes. One can only hope.