I don't think the over all plot of Korra was bad, the execution really needed a lot of work, however most can be fixed with small changes.
Mainly re-dubs should do the trick, mainly in the revised version of "Spirit of the competition" Tarrlok, Amon and Beifong could get into spates on the radio, cutting down on the animation budget.
Given what is going on right now, the voice actors don't even need to be brought into the recording studio. Avatar: the last bender had some of the child actors record their lines at home in some cases given they had to go to school still.
Small animation changes
Most of the action scenes were done well just like in Avatar, but some non-action scenes really need fixing.
The scene were the air bender family first sees the bunny dragon fly spirits was the worst animation in the whole franchise, it looked like someone glued pictures of them onto Popsicle sticks and waved them around on a background.
Korra’s overall arc
At the beginning, instead of Korra being discovered by the White Lotus it should start out with Korra's parents and Sokka having stopped the Red Lotus from kidnapping Korra
Make it that Korra was old enough to actually barely remember what happened (say 4 or 5 years old), this way she would traumatized by it.
This would not only give a better framing to the series but explain why growing up with this experience would be why she is aggressive, despite having loving parents.
There doesn't even need to be a ton of extra animation: there could be a news broadcast on the radio say what happened
Mako meeting Asaami
Instead of her running Mako over with her bike, she would be driving around a bad part of town without realizing and someone tries to scam her and he warns her.
This could also be a good comedic scene.
To thank Mako for this she takes him out for dinner.
Korra meeing Mako and Asaami
Given Asaami is the daughter of a well known person, Korra could have contacted her for whatever reason in the past and they could write back and forth for years.
When Korra meets Asaami in person, the two don't know if they can hit it off still at first, this would be a good parallel with internet friends
Korra said she only had Naga as a friend, this would give her even more motivation to go to Republic City, too see a friend she never actually met.
Korra could have also been shown to listening to Mako on the radio back at the South Pole and followed updates on him in the newspaper. Mako along with Asaami would also be the only contact with people Korra's age while stuck at the North Pole, this would better devolp why she ends up having crushes on both of them. (Just a thought...).
(Speaking of which reduce Korra and Asaami’s relationship to a subplot in the comics, just like all the other romances in the series like I suggested in part 2).
Korra should bump into Mako in the first episode and they fight off the thugs together
Korra asks for his autograph after the fight, but is interrupted from getting arrested
Mako shouts to her that they can meet up later after a match at the pro-bending arena.
Then the next episode would be Korra meeting Bolin, Mako wouldn't tell him Korra was the avatar at first as he gets overly excited
Korra maybe didn't even realize Mako had a brother.
That perfectly Segways into:
Mako and Bolin's development
Mako is more of the star and Bolin lives in his shadow or isn't noticed as much.
Bolin should still be happy-go-lucky, but be shown to be more sensitive
In my revision of the worst episode, Korra would tell Bolin she doesn't know what she wants and he takes rejection poorly.
Keep the scene with Bolin getting drunk from eating, where he accuses Mako of "stealing" Korra from him, showing this isn't the first time this sort of thing has happened
While Bolin appreciates what Mako does for him, he could also secretly be a bit resentful of him and has to overcome this at the end of the first season. Bolin's hidden resentment would also be a good parallel with Amon and Tarrlok and also Korra's father and uncle.
Season 2 Bolin would have to gain more confidence
Women treat Bolin like crap in season 2, if they have to keep this make it part of his arc.
The actress that treated Bolin badly on set, when he asked her out saying "you're confusing Ginger the actress with Ginger the character, Ginger loves Nuctuck not Bolin"
Bolin should work to gain more confidence in himself. Ginger liked Bolin after he saves the arena, but instead of him accepting her kiss he should throw her words back in her face telling her "you’re confusing Bolin the actor for Nucktuck the character, its Nucktuck that loves Ginger not Bolin''
Bolin got involved with Korra's Yandere cosine, Eska. It was made obvious that Eska was abusive at first, that's not how toxic relationships start out
While there may be red flags the abused might not have realized, but abuse gets worse over time.
Go back and make it seem at first that Eska is just really shy/socially awkward , then becomes more controlling as the relationship goes on. Keep the scene where Bolin and Eska make amends, but expand on it and have her realize and explain to Bolin why she treated him horribly.
Change Desna’s role/relationship to Eska
There was really no point to Eska having a brother, as the two were basically the same character.
Either find a use for Desna or just get rid of him and give whatever lines he had to Eska.
If you bother keeping Desna, he should be a family servant not Eska's brother.
Desna being the family servant would also explain Eska's controlling behavior:
he would have no choice but to follow orders and would be the only person she considered a friend for years.
This in combination with her rotten father would lead to Eska believing all relationships function like this.
Change Desna's design, but keep the color scheme to show his relationship with Eska
Desna as Eska's brother has only made him the butt of jokes:
One joke when we first see Desna and Eska, and Bolin thinking Desna is a girl,
hilarious and original.
The other joke being in season four as the butt of an incest joke, which admittingly was actually kind of funny, but not worth keeping his character for.
The joke was that Desna and Eska were staying at a hotel, but their room only had one be
Eska then says, "That's ok...Desna can sleep in the bath tub"
While I thought this joke was funny for the bait and switch, it reinforces that Eska sees Desna more like a servant than a brother (he could sleep on the floor at least you jerk ).
This joke could still be kept, but with a slightly different context.
Raava and Vatu
Make Vatuu and Raava one in the same character-
In science its explained that there's actually no such thing as darkness, just a lack of light. Generic doomsday villains in of themselves aren't a bad thing, as long as the rest of the writing is strong enough.
The generic doomsday villain doesn't work for Vatuu, him being a big spooky shadow defeats the philosophies of Avatar: there's no such thing as good and evil, someone or something is out of balance.
Have Raava be at constant war with this terrible side for herself and make it be more Lovecraftiaan. Have the spirits "go dark" without Raava even realizing that's what her state is causing.
Regardless if Vatuu is kept or is just a bad part of Raava, make him a silent antagonist, his mere presence should be frighting enough.
I feel this suggestion might be more of a subjective opinion, I would like to hear everyone else's thoughts on this.
I do know people felt that "Beginnings" retconned the origins of the avatar:
The lion turtle saying to Aang that "In the age before the Avatar, people bent the energy within themselves" then in "Beginnings" have the lion turtles give people ability.
However I don't this is an inherent contradiction.
People could have still gotten these energy bending powers from the lion turtles and the "era before" doesn't mean it was immediately before the avatar existed.
Shakespeare was born after the fall of the Roman Empire, that doesn't mean he was born as soon as the Roman Empire fell.
The origins of the avatar could be expanded upon more to clear up any confusions.
What I think everyone could agree on however:
Make the stakes for Vatuu taking over in season 2 actually clear-
Have the "ten thousand years of darkness" be just that, ten thousand years of darkness that destroys all life. That's it, there's no need to make it anymore complicated than that.
Other elements
The detectives from season:
Get rid of them completely, I mean it.
These were the two most useless characters in the whole franchise Avatar franchise.
Just like with Desna and Eska, they’re the same character.
I can understand Mako needing an obstacle, but these two were very poor ones. Mako being inexperienced as a police officer and making mistakes should have been enough on its own.
The detectives were a big part of season 2 being needlessly mean spirited, cut them and give their screen time to Lin instead, Lin needed a lot more development.
Fix Pro-bending
I ripped this next part from part 2, but I'll repeat it because this is the best idea I have.
"Some fans have taken issue with the pro-bending being too controlled of an environment, which took a lot of the tension away in the plot. Funny I'm not a sports fan either, but even I know that lacrosse and Football (American Football to the rest of the world) are dangerous sports even with with safety equipment. Back in the 1920s when Korra lives, these sports were even more brutal with equipment that could break much easier than what players have now. Have this episode (and the others) show equipment being blasted into pieces from the force of the elements during the game, making injures players get more serious. Speaking of which if Korra really had make a stupid move on Mako, maybe she could have admitted she had feelings for him while she had a concussion"
I think this would the best time to get a re-cut, this series isn't as old as other franchises, and Mike and Bryan themselves have often joked about "George Lucusing" things, they should take that idea seriously with The Legend of Korra.