"Book: Air" was a very common name for fan sequels, in keeping with the Theme Naming of the show's seasons, as it was the only main element remaining that did not have its own season. The first season of Korra is titled "Book One: Air".
When the series was initially announced as The Last Airbender: The Legend Of Korra, some fans rejected the Artifact Title and its association with the much-disliked film adaptation, instead referring to the series as The Legend Of Korra. Official sources soon followed suit.
Two of Aang and Katara's kids are named Kya and Bumi, after Katara's mother and Aang's oldest friend. Fanfiction writers named hypothetical daughters of Katara "Kya" years before The Legend of Korra was ever announced.
Brolin for Bolin. Even more appropriate now that we know about his lady-killing ways.
Mama!Mako is becoming quite popular on account of Mako's doting and protective ways.
Sparkly Sparkly Bush Man for the park-dwelling vagabond Korra meets in the first episode (after Sokka's temporary name for Combustion Man of the original series). Secondary sources later revealed his real name as Gommu.
Korralition or Krew for the new iteration of Team Avatar, as their in-universe team name, the Fire Ferrets, wasn't known to the fandom until early March 2012.
Spider-Lin for Lin Beifong after her actions in Episode 6.
Many of the characters have earned themselves superhero themed nicknames. Lin has also been dubbed "Iron Lin" for the way she puts on her armor, and "Wonder Lin" both for being amazing and the fan theory that Toph created her via earthbending. General Iroh has earned the nickname "Iroh Man" after the finale, and Mako has been dubbed "Batmako" for his similarities to Batman in episode 3.
Big Daddies for the mecha tanks introduced in episode 7, especially due to the diving suit inspiration.
Korramobile for the Satomobile the Krew rides in when hunting Equalists in Episode 8 is somewhat popular given all the Batman undertones.
"Honora" for Zuko's daughter (the current firelord) who has yet to appear in the show.
Genius Bonus: Hiroshi Sato was based onHenry Ford, head of the Ford Automobile Manufacturing Company. Another fun fact about Ford was that he was rumored to have supported the Nazis. Take this into account with Hiroshi's character.
Episode 6: Lin was the third person in Tenzin and Pema's Love Triangle.
Episode 7: One of the Satos is working with the equalists. It's been a long time theory at least one of them is. Also, mechas. Super bonus points for those who saw the blue prints way back in episode 1. Also in-universe with Korra having eavesdropped.
Episode 9: Tarrlok is Yakone's son, and Amon has a means of protecting himself from bloodbending.
Thus far in the finale: Tarrlok is Amon's brother—his younger brother. Tumblr went on fire for that one.
And Amon is a bender—and a bloodbender.
Pretty much every major plot point was successfully predicted. It's only a matter of time before a waterbending Serial Killer named A. Firebender shows up.
Book Two: Spirits!
Name's the Same: There's a real person named Hiroshi Sato; a retired Japanese football player.
Playing Against Type: Steve Blum as Amon, Clancy Brown as Yakone, and Lance Henriksen as The Lieutenant are the only major exceptions so far. PJ Byrne starred in quite a few medical and/or legal dramas before being cast as Bolin. Mako's actor David Faustino was best known for his role on Married... with Children. Mindy Sterling (Lin Beifong) and Janet Varney (Korra) are best known for comedic roles. And while Dee Bradley Baker isn't new to voicing menacing villains such as Tarrlok, it's certainly not something he's necessarily well known for.
Production Throwback: Naga the Polar Bear-Dog, Korra's animal guide and occasional transport for the new Team Avatar, was originally going to be Aang's waaaay back before the series was fully realized- sketches and concepts for this can be seen in the Avatar The Last Airbender official art-of book.
Promoted Fangirl: Janet Varney was a big fan of the original series even before auditioning for the role of Korra, needless to say she was stoked to hear that she'd booked the part.
Throw It In: A scene in "The Spirit of Competition" where Mako is angrily walking through a door and completely ignores a fan trying to say hi ("Hey Makooooo-oh") originated when someone drew a random extra standing next to Mako in the animatic and the episodes director thought he looked disappointed.
Working Title: First Avatar: Legend of Korra, then The Last Airbender: Legend of Korra. The "The Last Airbender" Artifact Title was ultimately dropped, while the "Avatar" supertitle was retained for overseas markets.