The Legend of Korra/WMG/the Setting

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

There will be a hidden Airbending culture
Just like the Sun Warriors in Avatar: The Last Airbender, the will be a tribe of ancient airbenders hidden. They will be called the Sky Barbarians, they will be based off the Mongols, they will have canadian accents (there's waterbending rednecks, so why not?), and they will have been plotting world domination for decades.

They will invade Republic City at some point. They will be the polar opposite of the Air Nomads, preffering violence over everything else and using all dangerous airbending techniques like tornados, hurricanes, choking people, venemous-gas/fart bending, soundbending and lightbending.

As it turns out, Korra will have a dillema. To save the world, she'd have to commit genocide.

The invention of guns will appear

 * Mostly it'll be flintlock pistols, rifles, muskets, and blunderbass, in order to give power to non-benders. The heroes will all be gunhaters naturally.
 * Possible. There was gunpowder in the original series, as well as some prototypical cannons in the third season. Guns would be a natural development from those.
 * Who says they should hate guns? Benders may not use guns because they don't NEED guns! I always kind of liked to picture a hard-core Sokka with a shotgun (of the blunderbuss variety.) Maybe the normals of this season will pack heat!
 * They could hate guns because of corrupt militias that use guns to massacre civilians or benders. Arrows, swords, and other such weaponry (and bending too) require training and discipline, and thus much time and money to create armies with. Mass-produced guns mean instant armies of whoever happens to be on hand. Anti-bender revolts could get bloody.
 * That's a silly generalization. You mass distribute guns -- especially older, primitive guns -- without any training or discipline and you end up blowing yourself up. Just because guns are not as flashy as non-firearm weaponry doesn't mean there's no training or discipline involved. By that logic they should also hate spears or javelins since they are also easily mass produced, required relatively less training compared to swords, and far outranged swords. And, you forget that corrupt militias have already been using bending and other non-firearms to oppress people (remember that episode with Zuko defending the town) for far longer. Lastly, guns = instant armies may be true now because of modern industry and guns like the Kalashnikov, but any guns likely to be developed in this time will be like the older arquebuses. Not to mention the need to manufacture gunpowder and ammo. And given that any gun industry would be in its infancy given that there wasn't one before, they're still at least years away from any sort of large-scale mass production, much less "instant armies".
 * Gun might have existed in the Gaang's time. The tech we see during the Gaang's time implies they had to have had the possibility of such weapons. They might have just not been that pronounced for various reasons. Namely like those stated above, the guns of Gaang's time were too unstable for mass use and the high bender count in armies would look down on such weaponry. The Anti-Bender mentality mixed with more advanced and stabler weapons could very well allow for the anti-bender forces to take up guns or refine gun related tech into guns to equalize the common man against the "magical" benders.
 * Although there is argument for both sides, I doubt it. Having guns would continue the creator's habit of introducing nuanced, mature subjects. It would also be consistent with the probable theme of technological advancement. Lastly, it is a good way to even the odds between benders and common non-benders, although low-level earthbenders would obviously have it easier than low-level airbenders. That said, guns would be more difficult to pull off. American cartoons are bound to certain expectations, far more so than Japanese ones. I seriously doubt we'd see anyone killed or crippled onscreen by gunfire. Because of that, it would be very easy for the gun-wielding bad guys to degenerate into Imperial Stormtrooper Marksmanship Academy, which would make them nonthreatening. As much as I would like to see it, I don't think the creators would risk that when, in the original, there were several threatening nonbenders, although admittedly they were in the top three percent.
 * One thing you have to remember is the style of gun that they would have had are very very very inaccurate. They were really only effective with massed soldiers and very short distances. It really wasn't until the late 1700's that very accurate guns cam, even then it took trained marksmen to be accurate over a dozen yards.
 * Now that the show is actually out (I assume this guess was made before it aired), the setting has turned out to be a lot more 20's-like than was initially believed; considering the level of technology we've seen, something like a revolver or even a Tommy gun wouldn't seem too out of place.

Despite the high technology, guns will not' make an appearance
The creators want to keep the show kid friendly and know it would be too much of a Game Breaker in terms of story telling.
 * Four words: Batman the Animated Series.
 * Considering that the Republic City police force mainly uses Metalbending, guns wouldn't be as much of a Game Breaker as you'd think.
 * Indeed, guns may not appear in any significant capacity, because any Metalbender could crush them into pieces of junk metal without effort. And since they make the bulk of the police force, any criminal relying on a firearm would be apprehended in a flash.
 * Indeed, that may be why metalbenders are the police, and would account for why there aren't any guns. Even if they existed, if your weapon is easily destroyed by the police, chi-blocking techniques would be much more effective and therefore the weapon of choice for non-benders.
 * Might be confirmed since
 * What does Batman the Animated Series have to do with no guns? There were plenty of guns in the show.

Going with the gangster theory, the show will potentially involve immortals.
Hey, taking place in a 20's-esque setting. Although there is the potential of an overload of awesome.

The play performed by the Ember Island Players has gone on to become an all-time classic, with the original ending substituted for the actual ending of Aang beating Ozai.
It's a national treasure of the Fire Nation and is performed all around the world.
 * ... That's just silly enough to work.
 * Is there an on-ice version? A few waterbenders and a bunch of ice skates here and there and you're all set.
 * A musical on-ice version.
 * No way. Fire Lord Zuko would never allow the play to leave Ember Island, let alone become a "national treasure of the Fire Nation."
 * Yeah, but you've seen how many people go to those things, even if just because they're So Bad Its Good. Besides, I'm sure Toph would have a word with her ol' buddy if he tried to cancel it...
 * The troupe could have easily changed the show in order to look better in the eyes of their ruler. In Shakespearean times, playwrights sucked up to the royalty through their scripts all the time.
 * Given, however unless Zuko bans it or sets the playwrights on fire they may not realize how irritated he is over the whole thing. I can see him and/or Mai trying to set the record straight via official histories and memoirs if they have the time but....
 * It will probably be a silly, campy Rocky Horror style show.

A lot of waterbenders will function as doctors.
And boy, they will cost much. Also, imagine what kind of sickness they could cure if they're using actual medicine instead of water. It worked so well and fast with regular water! They will certainly cost a lot. Maybe Katara opened up a medical school when she was younger?
 * In episode 7, Tahno mentioned going to several healers after . So that could be taken as evidence that waterbenders set up shop for their healing services.

Silent film now exists in the Avatarverse.
Well, this is apparently set in their equivalent to the twenties, so why not?
 * And "The Boy In The Iceberg" with a Revised Ending, will be the most popular picture. Korra and Tenzin will each either love it or hate it.
 * Or fall over laughing at the absurdity.

One word: Steambenders
Hey, cultural melting-pot city, fantastic, almost unimaginable technological progress...perfect setup for the formation of a new form of bending. A special subform at the very least, or an entirely new "hybrid" discipline at most/worst. Plus, goggles. Gotta have the brass goggles.
 * Steam is just evaporated water, so this is pretty likely. Like how "sandbenders" were just earthbenders who had better control of sand than the average earthbender, a steambender would just be a waterbender with better control of steam than the average waterbender.
 * That is not a new bending type, they could bend it in the original, it is waterbending.
 * Aang and Katara shaped clouds to manipulate the fortuneteller, and Aang harvested a cloud for drinking water.
 * But maybe they could be airbenders...
 * Katara specifically created steam as a cover on the ship at the beginning of the third season and Korra had no problem doing this in "The Revelation." It was already established in the original series that Waterbenders can bend all states of water though like powering plants with lightning generation it still could/has applications in an urban setting.
 * If this is the case, I would be happy if there was one Steambender voiced by Richard Horvitz...
 * I will second this if it's a Raz expy.
 * There will be a guild of Firebenders and Waterbenders who are bound to create and improve steam-based technology. And their leader will be a red dragon.

Iroh's Jasmine Dragon will be the new Starbucks.
Wouldn't it be funny to see people walking around Republic City drinking tea from Jasmine Dragon labeled paper cups?
 * I support this
 * ...I would buy from Irohbucks... (As long as is the logo.)

The Fire Nation will be a rustbelt and declining power.

 * All empires and superpowers eventually decline, for one reason or another. By Korra's era, the Fire Nation will no longer be a superpower. Why? Because the newly industrialized Earth Kingdom (and possible Water Tribes) will have a leg up on them when it comes to their infrastructure, being able to apply the best practices learned during the Fire Nation's industrialization to their own countries, as well as producing homegrown innovations in the means of production. The EK and WT also won't be saddled with the legacy of a hundred years of industrial civilization. Over time, this means that other countries will be able to out-compete the Fire Nation when it comes to producing things like textiles, cheap steel, and the other goods of an industrial society. While Zuko and his descendants will attempt to combat their nation's decline, lingering cultural chauvinism will prevent it from adopting 'foreign' ideas. Also, Fire Nation workers simply won't be able to compete with the sheer manpower supply in an industrializing Earth Kingdom; they'll work longer for cheaper because there's ten other Earth Kingdomers in line to take their job.
 * This seems probable. The end of the war is most likely going to hit the Fire Nation hard, what with the reparations and recessions that accompany the end of war economies. While it's possible that they can bounce back from this (compare Japan after World War 2 to Japan today), they will have lost several of their advantages, as you said. Also, from what we've heard of the interquel comics, we know that there's another major Fire Nation conflict bad enough for Aang to get involved. Depending on what that conflict is (say, a Civil War against Zuko by Ozai sympathizers), that's most likely going to compound their problems.
 * I could see this being the case if the Fire Nation was still The Empire, but seeing as it was Zuko and Aang that started The Republic, this seems a lot less likely. The proliferation of Steampunk technology would have been a collaboration of the FN and the Mechanist's group, at least for the first few years. By the time time the EK could compete with them again, there would no more EK or FN, just the Republic. As for reparations, the leaders of all the nations were at that point on good terms with the Gaang, and seeing as Fire Lord Zuko was a member, I really don't see them going all Treaty of Versailles on the FN's Germany.

Agni Kai will be illegal in Republic City. It will also be seen by many as an anachronism.
Historically, as societies become less feudal and the rule of law becomes more important, judicial combat declines and is eventually outlawed. The agni kai will be treated the same way in Republic City. The popular perception of it will be less "means to avenge wounded honor" and more "excuse for vigilante brutality". They'll still occasionally be fought, mind you, but they'll be relegated to the shadows, like street racing is.
 * Of course Mako is eventually going to be challenged to one, because it'll be a good way to milk drama about multicultural families and to illustrate to Korra (and the viewers) how cultural customs are different in Republic City compared to, say, the Fire Nation with how firebenders are expected to behave.

We will see the rise of new spirits.
There are already forest and nature spirits, and so machine and city spirits will arise, the city spirits' rapid expansion will clash with the nature spirits, forcing Korra to become more spiritual to mediate a compromise, meanwhile machine spirits will arise and will be prayed to similarly to the Adeptus Mechnaicus in Warhammer 40000 as well as delivering fortune to those who properly maintain their machines.
 * I kinda want to marry this idea.

Benders in Republic City will be expected by custom or law to wearing something identifying themselves and their element.
With the end of color-coded clothing, and the intermixing of peoples meaning eye/skin color aren't even useful as a rough indicator in Republic City of someone's elemental background, benders will start wearing tokens to show their alignment. This will take two forms. First, something like a colored scarf (a la Mako's neck one and Bolin's waist one). Second, it might be a piece of jewelry, like a sun or moon necklace. But whatever the case, it'll be something worn on top of a regular outfit.
 * May be a coincidence, but of the the three bender thugs Korra fights in the trailer and leaked footage two wear an article of clothing with their bending colors (the firebender and the earthbender) while the waterbender doesn't. It may be more of a status symbol than a legal thing, though I wouldn't doubt that benders have to be licensed to bend within city limits like having a driver's license. Korra may have to get three licenses for the bending arts she has mastered. with comic results.
 * Let's expand on that further: maybe the bender license thing was introduced as an appeasement to the anti-benders. It sounds like something they'd want-being able to track of every bender in the city. In addition, bending in fights is illegal in certain parts of the city; bending in school gyms or special arenas or non-violent bending (healing with waterbending, building stuff with earthbending, lighting a match with firebending) would be alright but using bending to beat people up, like Korra did, is illegal. Of course, this does more harm than good since the benders are bound to resent it.

There will be a countercultural element in the United Republic that mocks the traditional cultures of the Four Nations.
Alongside the bender/non-bender divide, there will be a conflict between the United Republic of Nations and the other four countries. Among the older generations, like Chief Beifong and Tenzin, the United Republic is a place that can be broken down into "tribes" that fall under the regular Four Nations. They'd see Mako as being a wayward son of the Fire Nation and Bolin as that to the Earth Kingdom. In contrast, the younger generations will simply see Mako and Bolin as citizens of the United Republic, and the fact that they bend is distinguishing in a way that whatever element they bend is not. As a rebellion against the "tribal" framework more traditionalist folks impose on them, Republic City's counterculture will purposefully adopt foreign elements as a way to mock them for being hollow symbols in a 'modern' era. For instance, a firebender might wear his hair in a Water Tribe wolf's tail, or a non-bender might wear clothes cut in the style of the Fire Lord. Some other countercultural types will take it a step further by doing such things as wearing their robes right over left and decorating with intentionally bad feng shui.

The United Republic of Nations has begun taking the Avatar's job.
The Fire Nation started the war during a period when the Avatar was too young and too inexperienced to stop them. As a result, the Republic was given special diplomatic and military powers so that they could act in the Avatar's stead while an Avatar is growing up and training. In keeping with the Romanticism Versus Enlightenment themes of the series, some of them are starting to wonder if the Avatar is now superfluous.
 * This very much seems like it's going to be the case. Avatar Korra as a figure doesn't receive nearly the same amount of regard as Aang did, at least in the first episode.

Pacifism will be a popular global philosophy.
Pacifism, once the domain of the Air Nomads, will have become a popular global philosophy owing to Aang's energybending of Ozai and his general behavior in ending the Hundred Years' War. However, the Avatar-verse's definition pacifism will be a bit different from what we think of it, as the go-to model will be Aang. So it'll be more of a militant or technical pacifism. Which isn't to say you won't have different ideological flavors. Killing will always be right out, but there will be disagreements about how far to take non-lethal injuries in a fight. Some will see energybending as an endorsement to cripple or maim (but not kill!) opponents to ensure they are no longer a threat. Others will see violence as only necessary in combat or training for combat, meaning some pacifists will protest against Pro-Bending as idealizing violence. Others will even view Aang's militant pacifism as an unhappy yet necessary concession to the dystopic state of the world at the time of his defrosting, but, now that the Hundred Years' War is long over and the world is largely peaceful, believe that strict pacifism should be practiced lest people slide down that slippery slope of justifying violence with technical pacifism.

The pro-bending championship is fixed.
In crime-ridden Republic City, organized crime just has too much money at stake when it comes to the big pro-bender championship. So the high-end matches are fixed; sometimes outright, sometimes just for the point spread. Except now that the Avatar is involved with a team aiming for the top, asking the Fire Ferrets to play along nicely isn't really an option, so the squeeze is gonna be put on Bolin or Mako to stay silent and comply with the mob's orders lest the other brother get hurt.
 * Well, ...yet.

The Boulder is one of the pioneers of pro-bending

 * Wow, just seeing that title prompted me to read it out loud in The Boulder-voice.

The Southern Water Tribe primarily uses a barter system.
Hence, why Korra thought she could still get by in Republic City and tries to purchase food without having any money, the logic being that there were plenty of favors she could trade, being the Avatar.

Mixed marriages are not as common as we've been led to believe.
Tenzin, the son of a Air Nomad father and a Water Tribeswoman mother, married a woman of an unknown ethnicity and Mako and Bolin are brothers of mixed heritage. But just because the war is over doesn't mean everyone's all buddy-buddy now. Outside United Republic, mixed marriages are still rare and frowned upon
 * We aren't even sure that Mako and Bolin are the product of a mixed marriage, they could just as easily be half-brothers.
 * "Half-brothers" means they still share a mother or share a father, which would still make one of them biracial. I think the word you are looking for is step-brothers. And I disagree because its too much detail for such a trivial point, unless its going to be plot-relevant. Also, Bolin and Mako's biracial heritage represents the cosmopolitan nature of the city. Why make that detail, then erase it later?

There will be new forms of Bending due to Aang's Energybending
He'll eventually give Ozai's bending to a bender of a different element after being pressured into an experiment. After a huge success wherein the two forms of bending combine (think lavabending if it's an Earthbender for example) instead of being controlled separately (because only the Avatar can do that), other Benders line up to have their powers mixed and matched. Eventually hundreds of Bending forms are spawned and taught around the world, even to nonbenders who are training to receive the ability to bend from Aang (cause you know, he can do that with Energybending). If this happens it'll probably be an attempt at recovering from the [very well done] Ass Pull that is Energybending, but I'll be damned if it won't be awesome all the same.

The glowy light show that Aang pulled off with Energybending? That's just what it looks like from the perspective of the bender and their target.
Witnesses just see it without the light show, akin to what Amon did.
 * Seemingly confirmed by "Out of the Past" - when Aang depowered Yakone, there was no light show.

The gangs of the city stem from varying post-war attitudes.
We've got the Agni Kais, the Red Monsoons, and the Triple Threat Triad. To go in reverse order, the Triple Threats are (as we know) made up of allied earth, water, and fire benders. Clearly, then, the founding firebender didn't have any issues with fraternizing with people from other nations, and would have likely been considered a sympathizer by the Fire Nation during the war. The Red Monsoons is a step away, being allies of water and fire benders. The founders, for some reason, didn't like earthbenders. The one with the biggest red flag are the Agni Kais. Just from their name, it's pretty obvious that they're some sort of Fire Nation nationalist or supremacist group.

The other gangs, then, would be varying mixes of earthbenders and waterbenders.

The Equalists will create their own monster.
Ok, there was speculation above that this series is just a set-up for another series that will parallel WWII. However, there's one problem with that: The Equalists seem to represent Communism (an extreme version of "equality" that leaves no room for accomplishment through natural talent), and we weren't fighting Communists in WWII, we were fighting Nazis...but wait! Prior to Hitler's rise, there was a strong Communist presence in Germany...in fact, many people voted for Hitler who otherwise wouldn't have been, because they thought he was better than Communism! In the same way, the Equalists will lead to the rise of a faction dedicated to the extermination of all non-Benders, forcing the sequel series (of which Korra will still be the star, there's no way they'd have time for her to die, and the next Avatar to grow up) to fight a war both for freedom, and for the very survival of non-Benders.
 * Tarrlok would be the most likely candidate to launch a fascist takeover. He favors violent methods to stop the Equalists without addressing their underlying grievances, Tenzin says he's greedy for power, and he seems to have the other three Council members in his pocket.

The 36 Levels of Airbending will finally get explained or shown in full
Unlike the other bending arts, Airbending specifically had a series of trials one had to undergo to affirm their mastery. Aang being the brilliant little punk he was, was so good his invention of his signature Air Scooter promoted him to Master one tier early; marking him as the youngest official 'Master' level Airbender in history- and coincidentally, relieves the need to display his native training and focus on the training of the following Arts. Perhaps during Korra's ongoing lessons with Tenzin we will finally see what it is that these 36 Levels are, and what was the last step Aang didn't have to do. Heck, it'd be really cool to see her do as her predecessor and make a distinct technique all her own.
 * There have been insinuations that the last step in Airbending mastery is devising a new Airbending technique. Since Aang already fulfilled that...

The "Advanced Airbending" will be Sound
Each of the other elements have a sort of "advanced" version. Water becomes healing, fire becomes lightning, and earth becomes metal. Korra, Tenzin, or one of Tenzin's kids will advance Airbending to the point where they can bend sound.

Alternatively, the advanced version will be Vacuumbending
Basically, the ability to create vacuums, which can be used to pull air into a concentrated form to be shot out(air bullets, or even air blades), and it could be used defensively by sucking in attacks thrown by other Benders. It could also be used to suffocate people by creating a vacuum field around the victim which sucks the air away.

The Airbender population will not be replenished solely by Tenzin and his family.
Through spiritual enlightenment and practice, airbenders will begin to be borne spontaneously to nonbender Air Acolyte couples.
 * I support this theory. While the Fire Nation killed all the Airbenders, it just means they killed all the ACTIVE Airbenders. Doesn't mean there can't be Airbender descendants who don't have the Airbending ability but do carry the genetic potential for it.
 * Especially considering that Katara is evidence that two non-benders can have a bender child. We know that Katara and Aang had two other kids who, while not airbenders, do carry the airbender genes. It's highly possible that Kya and Bumi could have airbending children.

Benders will adopt Equalist weapons.
Everyone can have the power of a chi-blocker in the palm of their hand... including benders. Imagine waterbenders electrifying their water whips, or metalbenders with a shootable tazer built into a glove. Not to mention dual-wielders like Zuko who will combine their bending and their shock glove to have varied short- and long-range attacks.

It is illegal to bloodbend in Republic City.
Given Yakone's work bloodbending 42 years ago, it isn't much of a stretch laws were enacted against this form of bending being used due to its massive cruelty potential. That is why Tarrlok asked his secretary if the other members left before dismissing her so nobody would catch him using it in combat against Korra.
 * In Harry Potter there were only three curses that resulted in an automatic life sentence: Imperio, which controls people, Crucio, which tortures (and is potentially fatal) and Avada Kedavra, which is an instant death spell. It wouldn't be surprising for the Avatar world to see bloodbending (unless used to save somebody's life) the same way that the Ministry of Magic sees Imperio.
 * What makes it disturbing is that bloodbending is able to act as all three of those spells.
 * Confirmed in "Out of the Past".

In another hundred years, pro-bending will have four-man teams, with the fourth member either an airbender or a nonbender
If the guess about the air acolytes getting airbender children is right, there will probably be a few hundred airbenders running around in a hundred years. It would be hard for the pro-bending sport to ignore that. However, even with a few hundred airbenders, there likely won't be enough willing to join the competition to require one in every team. Instead, teams can have either an airbender or a non-bender. In order to maintain a bit of balance, non-benders will be allowed to use a shield and a stick with a weighted end. Airbenders will get limited in that they can't spend more than a second off the ground, as well as not being allowed to channel air for more than a second (like waterbenders)

The Kyoshi warriors have or will learn their own unique style of waterbending
Currently, the kyoshi warriors do not appear to have any benders on their island. However, they do live nearby a mystical creature that spews water. Many of the bending arts were originally learned by observing animals. You do the math.
 * Likely jossed, as the people of Kyoshi Island were former earth kingdom citizens with Avatar Kyoshi herself representing earth.

Republic City is facing an Evil Versus Evil Scenario
As of Episode 8, we have The Good the Bad And The Evil. Korra and co. are the good. Between Amon and Tarrlok, Your Mileage May Vary on who's the bad and who's the evil.

There will be sky pirates.
We've seen that zeppelins are fairly common in and around Republic City. It's probable at least one got into the hands of some less-than-honorable people, who now go gallivanting around the world stealing whatever they want and escape in their zeppelin. As an added bonus, the Steampunk crew will be a mixture of water and fire benders.