The fact that water people are the first inhabitants and founders of Element City, with fire people being the last one to arrive, makes a lot of sense if you think about it. The first living beings (animal, plants, microorganisms…) on Earth on its primal stage were born in the ocean, until one day, thanks to evolution and many gap ages, some of them were able to adapt and live on the land, while other organisms even learned how to fly. Fire, on the other hand, is mostly present as result of many phenomena (volcanic activities, droughts, thunderstorms, etc.).
Speaking of evolution, fire is technically speaking the last element “discovered” by humanity, as the first men learned how to make one, and found out its potential as a means to protect themselves from harsh, cold weather and predators.
It's also implied Burnie (Bùrdì) and Cinder (Síddèr) are among the first fire immigrants to the city, given the city only notes the first three waves of immigrants, the earth-elemental who welcomes them into the city clearly isn't familiar with fire elementals very much and just gives them arbitrary names since he can't make heads-or-tails of their "fireish" language, as well as the racism they end up having to endure for several years, which is considerably lessened (though by no means gone) by the time Ember is an adult herself.
Similar to the four waves of immigration in the United States, there were four waves of immigration in Element City. Fire Elementals immigrating to Element City, would correlate with the 4th wave post 1965 immigration of Latinos/Asians.
Ember mentions to Clod in the beginning of the film to not let her dad "catch him again". Considering all Firetown residents are, well, fire (and Clod's home is in the Earth district), you'd think it's just Ember reminding him that her parents are wary of other elements besides Water (especially given how they were treated upon first arriving in Element City). However, if you look closely at Bernie's BANNED board, at the bottom left corner you can see a photo of an Earth Element kid. Who has similar features to Clod. If Ember reminding her father that "not all Water looks alike" tells us anything, Bernie seems to hold that same view towards Air and Earth too.
It's also a pun; Clod shouldn't let Bernie "catch" him again, because if he did, he'd probably set the little Earth guy on fire. Hence, catching on fire.
Despite the movie's themes on immigration and classism, the movie has No Antagonist, with the only real villainous characters being two unnamed water guys who harass the shop and are quickly dealt with by Ember. If anything, the antagonist of the film is the setting of Elemental City itself, with it being an active danger to Ember and her family by its very structure; there are rivers and dams everywhere, and Ember can't even go to Wade's house without substantial risk of getting extinguished. This still ties into the themes of class dynamics; as anyone who's dealt with it can tell you, one of the biggest issues with hierarchal systems doesn't come from mean individuals, but societal systems that don't allow certain groups to thrive (such as lack of healthcare, generational poverty, redlining, etc). Elemental City was built without considering the needs of Fire people, and it's apparent in every way that it's structured, which is why it doesn't account for putting them in danger. It also parallels the social model of disability.
One of Wade's relatives is his sibling Lake, who is nonbinary, and the rest of the family seem rather accepting of this fact. It's possible that it could be a difference in gender norms in the Elemental universe versus Real Life, but if gender expectations are the same as our own world, then it makes sense why Wade's family is very accepting. They overall seem to be Innocently Insensitive to Ember but are otherwise very respectful and seem progressive-leaning given their emotional openness, so it would check out that they're probably the same way towards Lake (although Lake probably has their own stories of their family not quite getting it, as well).
Lake is obviously genderfluid.
There are a few early hints that Ember wasn't meant to run the shop:
Ember is also noticeably very bad under pressure, when being asked a lot of infuriating questions or having to do something quickly, which, when running a store, is something that you have to do every day. It's no doubt that if she actually inherited it, she'd be miserable.
The few moments when Ember seems happy running the shop is when she's creating things, such as the glass lollipop and the charcoal balls. She later shows the same creative talents with the glass flower orb at the beach, the glass barrier for the dam, and the pitcher at Wade's family dinner.
When Ember is in the store by herself, it's always noisy and chaotic, and she's usually running around on fumes trying to keep things together. In contrast, all of her artistic creations take a careful amount of detail and patience, and the film even reduces the background chatter to allow more focus on them, showing that she had this talent from the start.
The movie focuses on four of the Aristotelian elements. However, the fifth element, Spirit, is readily apparent. Not only can the mother literally smell love, but it's a measurable force that neutralizes the interaction between Ember's fire and water. While regular elements are still dangerous to them, love balances them out.
There's a few moments where Ember, before doing something reckless or dangerous (such as getting into the bubble to dive, or doing some free-running to hop across floating debris), takes several short breaths in quick succession. In real life, fire needs oxygen to burn, and a few quick puffs of air can help stoke up a flame to make it stronger. So in other words, Ember is literally stoking herself up just before she's about to act.
When a Fire person dies, they do not leave behind a body, as shown with Ember's grandma. If someone decided to murder a Fire person, they could do it without leaving any evidence...
The same could rather easily be said of air people too. They're simply clouds and while we don't see one die on-screen, it's not hard imagine one that dies simply dissipates into well... air.
Wade's supposedly Absurd Phobia of sponges not only isn't as ridiculous as it's painted, as a small child was left trapped in a small space for a good while (even if we assume he's exaggerating being stuck for hours) but highlights an unsettling possibility. If it's that easy for a Water person to get sucked into a sponge, it's entirely possible for an easy kidnapping method for Waters to exist; throw a sponge at them and/or force them to touch one, let them get fully sucked into the sponge, wring them out into a sealed container and stick the container on a shelf while you write off the ransom note. As for how they'd be disposed of once they're no longer useful? Either dump them far out in the middle of the ocean or evaporate them somewhere without porous building materials so they can't recondense properly.
