Final Fantasy has, for a long time, incorporated mature themes and brutal death behind a veil of color and fun. The death usually happens to non-main cast members, so that it feels distant, and in the earliest iterations of Final Fantasy, it was usually animated like a cartoon wherein the people dying either died in a huge explosion or off screen entirely.

The darkest Final Fantasy to date is still probably Final Fantasy II. It involves the Emperor Mateus of Palamecia summoning monsters from Hell in order to take over the world. You play four orphans who are orphans because their family and town are assaulted by the Empire and its Hell monsters. They themselves are nearly killed in the opening of the game, and are saved by the rebellion and the series' first WHM character. They go on to assist the rebellion, but in doing so lead several people to their deaths... they save many more in the end when they finally stop the Emperor, but not before every major town in the game is bombed by his Dreadnought nor before the Dragoons of Deist are genocided along with their wyverns. The Empire also genocides the kingdom of Kashuan. Also, after they blow up the Empire's dreadnought, the Emperor himself forms a tornado with magic that levitates a castle inside, which he uses to destroy every major town except for Mysidia and Fynn with, thus killing any residual populace that weren't killed in the dreadnought's bombing run. So you do stop him in the end, freeing the world from his Hellish conquest, but it's basically a pyrrhic victory. On top of that, there aren't really any comedic break anywhere in the story, and there are no mini-game chains to distract you from all the death. All the death may not impact the way it would in a properly fleshed out story, but the tone of the game is bleak. I actually got a lot of FF2 vibes out of the XVI trailer, but maybe that's just me.

FFIV is where the series staple of having horrific stuff happen, but then hitting you with personal drama or funny town happenings starts. It kicks off with the main character killing a ton of mages and stealing their town's Water Crystal. He's stripped of his command for questioning the king about it, but then you get a cool DRG bro levied on you, a hot WHM GF, and an eccentric Airship Engineer patting you on the back. In town a dancing girl will strip for you, but the spectacle is limited by the graphics. After that... you accidentally genocide a town of Summoners. The game continues on like that, dipping up and down, where the story tone is dark, but the inbetween bits and towns are full of silly events and dancers and porno mags etc. This sort of structure carried forward into every future Final Fantasy in some form or another, the effectiveness on the comedy bits ability to distract you from the dread varying title to title.

With XVI's trailer we're shown a fairly grim perspective. Seems like a tough nuts sort of world, and it goes back to older FFs, where if there is any advanced technology it's going to be specifically locked in a location and no characters are going to be using it. So I think they're basically going for living out Square's ambitious vision for Final Fantasy II, now that it can be fully realized by modern gaming technology. We're shown in the trailer that there's a sort of secondary citizens or "subhuman" caste for society, with the cheek branded soldiers. They showed blood from a decapitation in the trailer splatter over a little boy's face. Ifrit seemed to be smooshing people into blood splatters. I think what they're going for with XVI is to make the darkness that's always been in Final Fantasy stand out. I don't think there will be a lot of mini-games and comedic distractions.

Rather than bottling dark fantasy as high fantasy, I think XVI is going to be mostly dark fantasy. It's speculation based off of the trailer, but I will be surprised if XVI feels light and airy. Though I'm sure it will come with its own subset of memes from the voice acted segments being taken out of context and stuff.