Part of me hopes Book 4 will take place years after Book 3, with Jinora grown up and Korra been given a long, long time to recover.
This would actually be incredibly interesting. If Korra and her gang were around the same age as Aang and his crew in the flashbacks in Season 1. This would give them a wide space to change the characters and have them get some off screen growth (Korra not as hot headed ect.). Sadly, there's no chance this would happen.
On a different note, I found the ending to be good but not great. Certainly better than the catastrophe that was S2 finale. Zaheer's plan was actually pretty smart, and the fights between his group and everyone else was good, though Mako and Bolin were fighting at a level that they shouldn't have been imo. Mako taking out the water bender with lightning made sense because it was not a win based on bending skill but rather smarts and using a move that is super effective against your opponent.
Bolin is still terrible, and is IMO a complete waste of time as he adds literally nothing to the show except waste screentime with terrible "humor". His lava bending isn't worth the little he does, and I think he actually makes the show weaker because the writers feel obligated to keep him along because of reasons, thus he is forced to stick around to be terrible comic relief. I even actually liked Bolin's character in S1 when he actually had some relevance to the plot.
My biggest problem with Korra and her group this entire series is that they don't do anything, things happen to them. At almost all points of ATLA, Aang and his group are doing stuff to achieve a goal. They are always a step ahead of a majority of the bad guys (minus Azula) and they almost always take the initiative (saving Ba Sing Se, trying to take out the Fire Lord during the eclipse). While Korra and her group are constantly outsmarted and outmaneuvered by everyone. There is very little evidence that Korra's group of competent at all since they almost never have an advantage on their opponents. I think the biggest factor in this is the fact that in ATLA the gang had time to have more minor villains whom they can outsmart/beat up, which in turn makes villans like Azula more badass since she is always a step ahead of a clearly competent group of powerful benders (with a decent strategist). On the other hand we have Korra's group who almost always gets their asses handed to them in fights, and the bad guys run circles around them and make them look like idiots for falling for the most obvious traps. At this point, seeing a bad guy outsmart Korra is the norm, and it makes it harder for me to even care about the plot as its pretty much Korras group getting pulled around at the villains leisure until they somehow figure out how to beat them during the finale.
I also felt that the completely messed up Zuko's cameo this season. He literally did nothing, and got completely jacked up by the evil Earth Bender guy. He even leaves at the most inopportune time to go "protect the Firelord" even though they know exactly where Zaheer and his gang are, and the only people who can stop him are going to meet him. This could be forgiven if Zuko at least appeared to be badass as he was made to look like a chump by 1 of the 4 members of Zaheers group, he would literally stand no chance if all 4 decided to show up and off the Firelord. The least they could have done is have him show up to the first fight with Zaheers group and scare them off cuz "oh shit its Zuko" or at least, you know, tip the balance in the good guys side for once, but nope he got wrecked by a guy Bolin and Mako can beat handily apparently.
The way that Zuko's return was handled makes me very iffy about them bringing back Toph, cause if they give her the same treatment as Zuko they might ruin Toph (my fav character) for me.
So at the end of this season, Tenzin's W/L ratio still sits at 0% and the good guy's in general sit at around 25% total (including random trash mobs like that bike gang in Ba Sing Se), and is even lower when it comes to fighting the main villains. Considering when Aangs group lost a fight it would be a considerable event, this goes back to my earlier point that they really need to make the good guys in Korra more effective or else we're just going to be watching the characters get drug around and beat up by the bad guys til they pull a win out of their ass in the finale.