The Legend of Korra/WMG/Canon Fodder and Call Backs

For theories about other subjects, see The Legend Of Korra Wild Mass Guessing Index.

The Blue Spirit will return
Someone new adopts the persona of the Blue Spirit, possibly to play both sides of the Bending Revolution, or to act as a neutral party in the revolution. Perhaps even multiple people using the Blue Spirit, all working together in Republic City.
 * In the original, it did sort of symbolize Zuko's rebellion. I could totally see it being used as a symbol of the non-bender rebellion, or maybe of an elite, ninja-like task force.
 * Alternatively, it will be used by someone who needs to hide their identity at points...you know, like a bender in a city rampant with anti-bender sentiments? Maybe Korra will be the Blue Spirit.
 * Or maybe someone (Bumi? The unknown Fire Nation character voiced by Dante Basco?) will take up that identity to break Korra out of wherever she's being held prisoner, as a Shout-Out to the original series.

We'll finally find out what happened to Zuko's Mom.
If only in passing. "Katara: Well Jinora, it's an incredible tale -- Jinora: Really? Katara: Nothing happened at all. (Smiles)"
 * It seems like a must for the next Avatar series.
 * Or, we'll find out little details, but never find out EXACTLY what happened...
 * Or alternately, Ursa unknowingly caused the anti-bender rebellion.
 * I know it sounds crazy (because it is) but stay with me for a second. Many fans believe Ursa was exiled to the Earth Kingdom. In exile, she meets some non-bender kid that she helps out, a kid who saw his/her parents murdered by firebenders and thought all firebenders were evil. However, like the village in Zuko Alone, s/he will be tormented by earthbenders. But then, s/he meets Ursa, gets to know her, and after a long time, finds out that she's Fire Nation. After a HBSOD, s/he was come to the conclusion that being a bender, regardless of which element, made you evil and that the Fire Nation started the War solely because their evil firebending Fire Lord was crazy, because he was firebender and that the only true way to have peace would be to kick all the benders out of position of power. That kid grows up to be Amon. Poor Ursa will never know that befriending a little orphan would cause so much trouble.
 * Ikki! Why did you do that? Gran-Gran was gonna tell us the story!
 * We share your disappointment, Jinora.
 * It seems likely that Ozai didn't have Ursa killed, and that after the events of the first series Zuko went on a quest to find her. Before she is interrupted by Ikki, Katara says that the ultimate fate of Ursa is "an incredible tale". She wouldn't probably use such an expression if Zuko had merely found out Ozai had indeed executed Ursa after she killed Azulon. And the word "incredible" implies that Zuko didn't just find Ursa in some Fire Nation prison after he became the Fire Lord, but that something more extraordinary happened before he was reunited with her.
 * I can imagine that something like this would happen:

Sozin's Comet will return 30 years earlier than expected.
The comet came a little too close during the last approach and was pulled out of its orbit by the planet's gravity. The new trajectory will bring the comet back into alignment in 70 years instead of 100 years and nobody will see it coming. With no prepared plans to exploit the comet's bonus to firebending absolute chaos will ensue as firebenders compete for dominance.

Tom Tom will make an appearance
You remember Tom Tom right? No? You know, Mai's little brother?
 * Tom Tom is the grown mayor of Republic City who is close to retiring. He will still have an obsession with flying lemurs.

Tom Tom and Hope will be married and have a grand kid Korra's age
Because pairing two baby characters from two different nations years later and tying them into the story again is exactly the sort of thing Mike and Bryan would do.
 * I'll take it further: Mako and Bolin are Tom Tom and Hope's sons.
 * Or their grandsons. Either way, it's perfect.

Sokka's Sword will be a legendary artifact.
Made from a rare material? Check. Forged by the greatest swordsmith in the world? Check. Belonged to one of the heroes that saved the world? Check. There will be legends about that weapon, it may even become a status symbol at some warrior's order.
 * That sword is durable enough to cut through metal without noticeably dulling, so who's to say it won't still be in that forest 70 years later.
 * Perhaps stuck in a stone...
 * And the Villain finds it.
 * Or someone who is on the protagonist's side (Bumi maybe) found it and refined the blade, even naming it "Sokkalibur".

The tsungi horn is the Avatar version of a Trumpet.
We'll get awesome jazz music and a continuity nod all in one!

There will be a dragon in the series.
Specifically a new born one, from that egg from 'The Fire-Bending Masters'. Because face it, that'd be awesome.
 * And it will be purple.
 * The egg will be found by Korra late in season one, and just for the Hell of it she'll set it on fire. Turns out this is exactly what's needed to hatch dragon eggs.
 * She already has a Polar Bear Dog, so this would put her on borderline beastmaster? Although it would be amusing for it live on Air Temple Island and awesomely terrifying against mooks. Naga's roar is enough to scare off Equalist chi-blockers.
 * Yes to the idea of beastmaster Korra. That would be epic. However, the dragon would be a baby for most of the series, so as to not overpower Korra. Also, cute factor.

Someone will say "Are you two so busy fighting that you cannot see your own ship has set sail?"
Iroh did say it should be a proverb

Airbending will finally have its own spin-off
Because Earth(metalbending), water(bloodbending), and fire(lightning-bending) had their own but not air.
 * Stormbending? (Aang would appreciate the irony, I'm sure)
 * I recall someone here saying it's sucking the air completely out of one's lungs, an instant death. Which in my perspective, is much, MUCH, scarier than being taken your bending away.
 * And it would be ironic: that the most peaceful benders of all would develop a technique that's an insta-kill for anyone.
 * Fartbending!
 * Pressure bending. By excreting a great force over a small area, air bending can break stone with ease.

Metal cages used to contain Earthbenders in the old series will be a museum relic.
Unless the practice of metalbending is strictly secured by only police units, it would be safe to say that the cages occasionally used to snag earthbenders and hold them are obsolete. Even then, it's likely the latter would be true and be used as a callback since there should be some museum in Republic City.

"Yuan" notes have pictures of the Gaang on them.
Similar to how real-world nations put Presidents (and historically significant people) on money. Additionally, they're colored orange, red, blue and green for the respective person, for it's different denominations.
 * One concept for the Avatar Yuan has Aang on the front. So yes.

Zuko's entry onto the scene will involve Lightning redirection.
He hears that the City is in a bad way because of the Equalist uprising being in full swing. He enters the scene, an equalist with zapper gloves accosts him, and he casually grabs their electrified hand, redirects, and then zaps them away without much exertion at all.
 * And he is elderly to boot.
 * Also, his scar will catch on fire when he gets really mad.

Tahno will learn to fight with a boomerang.
he'll fight alongside Korra to beat Amon Because really, how cool would that be?
 * Oh my, I hope this happens. He is shown to have traits of the The Smart Guy being the most intelligent of the Wolfbats. The boomerang thing would mean he could embrace his water tribe heritage and prove even more badass.
 * This troper just had a mental image of Tahno with grenades!

Korra's Lame Comeback of "You're oppressing yourselves" will be the line that breaks Amon
Korra's comeback to the protester in the first episode sounded silly and poorly thought out, but it does make some sense if you compare Amon to Mako in that the former is holding onto that and is held back by the memory and its circumstances, being what breaks him. Even more so when Korra says it clearly with a serious tone.

Bloodbending will return to be used against the Equalists.
Either Korra or Tarrlok will learn bloodbending, use it to defeat Amon, then wreak havoc on the rest of the Equalist movement. From there the Equalists have even more reason to lash out against benders for the violation while benders have an excuse to go on a witch hunt over the de-bending.

The hippie nomads are still around.
They would have discovered Guru Pathik at some point and started eating onions and banana juice with him, thus lengthening their lifespans to coincide with the next avatar's.
 * Korra will seek them out to learn Energy Bending and hilariously frustrated by their laid back attitude. The episode will culminate with her learning to chill.

Mako will show he is able to bend with blue flames.
Either by harm done to Bolin or other means, he will enter a state of Tranquil Fury and fight with a cold demeanor to his opponent (either an Equalist or Season 2) and show he is able to conjure up blue flames if angered. Lightningbending is a common thing, so why not blue firebending among a few firebenders too?

Zuko will teach Korra swords.
He'll show up and be very unhappy with Korra, because she thinks she's nothing without her bending. He'll remind that she's the Avatar, and that bending the elements is only a part of what that means. Instructing her in swordsmenship would give a skill Amon can't take away, as well as help get her get the mindset for Air training. For bonus points, Zuko gives her Sokka's meteor sword.

The giant statue of Aang functions in the same way as the Avatar statues from the first series ...
Meaning its eyes will shine like a lighthouse when Korra first enters the Avatar State. That'll provide the clue Tenzin needs to hightail it over to the city and calm her down before she causes too much collateral damage. Or, Korra will have to return to the statue, the site of her greatest defeat so far, in order to commune with Avatar Aang as he first did with Roku. Or both.

Aang will make an awesome Brick Joke
Korra will go to Aang in search of answers for Avatar stuff. She'll ask questions that he can't answer, because it's the kind of thing that she has to answer on her own. Aang will remember how much he hated getting those answers from Roku, so he'll try to distract her... with the same marbles from episode three. After this fails, he'll give her the dissapointing answer.

Korra will learn to track and find people by spiritually locating them.
This one was so rarely used in the first series, it seems to be forgotten. Both Aang and Guru Pattik used this method to find someone by using someone that they were deeply connected to. Tenzin will likely show Korra this trick by spiritually aiding her find a member of the Krew that was kidnapped.

Korra will first converse with Aang on the Vernal Equinox.
Aang first conversed with Roku on the Winter Solstice, the beginning (or midpoint, depending on reckoning) of the season opposite Roku's native element. Korra, in turn, will do the same on the Vernal Equinox.

Azula will make her return as one of Korra's cell mates.
Korra's going somewhere where "no one will ever find" her, likely a high security prison. Whether Azula's gotten saner or not won't matter if she decides she belongs there.

Tarrlok got the Equalist items from the evidence at the Metalbending station to do his cover-up.
In the episode 9 preview, Tarrlok is doctoring the scene with a few items the Equalists use. Obviously, he is using the items the police and task force confiscated from arrests, bonus points if the glove was Asami's.

Amon is Koh.
It's why blood bending doesn't work on him. He is pursuing an ulterior motive with connection to the spirit world. This is his mortal form.

In Amon's mass debending, we will see closeups of several familiar benders.
Skoochy, Toza, Saikhan, and various other benders we seen in cameos or other scenes.

Yakone and Tarrlok's blood bending can't be performed on full moon because they're not actually drawing upon Yue to do it.
There are spirit world shenanigans afoot, and one of the spirits is giving them more power than they should naturally possess. Yue is not cool with that, so as her power grows Yakone's/Tarrlok's weakens.