For that matter, there's an understated element of horror that is a constant undercurrent built right into the universe. Besides the overt ones like being forced to question your own sanity (in Tifa's case) or actually being psychologically compromised (in Cloud's), or how ominous/malicious building a facsimile town with facsimile people is, the very premise of a single corporation that undisputedly rules an entire planet and controls every aspect of daily life (down to if you die that day) is inherently petrifying. Even just environmentally. People living amongst clouds of toxic pollution and whatever is dropped from above, by necessity re-purposing decrepit buses for homes and creating tools from scrap metal and refuse, is all very significant (and sufficiently depressing).
I'd like to see that significance being reinforced aesthetically. The slums are described as a world without rain and "without night or day," so why not have slum dwellers dressed drastically differently from their plate counterparts. Have them wear lights attached to their clothing--which are themselves patched together from rags--as both a sartorial statement and a practicality. Maybe you can't get past some areas without wearing the equivalent of a hazmat suit, and maybe certain sectors are unnavigable without night vision implements. Perhaps the people themselves should look paler than normal, and vaguely ghastly. Give me scenes where NPCs stress the scarcity of water, or background dialogue about the crime and suicide rates. Let me walk past (or even explore) synthetic sun rooms that the denizens have found necessary for their mental health. Moreover, the game is a cyberpunk dystopia (Midgar in that new trailer honestly recalls Blade Runner), so make it look one. We know and are told that Shin-Ra are manipulators of politics and media (which they control completely), and are actively trying to root out resistance, so why not illustrate how oppressive of an environment the slums are by adding not only the paramilitary occupation, but obvious signs of constant mass surveillance and ubiquitous screens looping nonstop propoganda. Because Shin-Ra certainly has the technology and wherewithal to do so.
Likewise, I'd love it if the protagonists' savagery were rightfully counterpointed with Shin-Ra's. I mean, of all FF games, VII probably has the most adult themes, and when you think of it, like 99% of the named characters in the game are almost wholly unsympathetic. The only reason they have any appeal at all is because your perspective is occluded to theirs alone. It's very likely that both Shin-Ra and AVALANCHE are villains in the eyes of much of the rest of the population at large (which one NPC actually implies). AVALANCHE for their part are not only destroying property, but also murdering innocent people, ending lives and livelihoods, and, by virtue of corporate entanglement, depriving people of food, water, shelter, and medical care. They offer no alternative social organisation, they are never shown providing supplies or aid to the people on whose behalf they claim to be fighting, and their methods and goals are entirely focused on anarchy, violence, and destruction. By any practical analysis, they really are trying to bring about the end of the world or at least civilisation. And when it's weighted this way, it really lends credence to the characters' own assertions that their motivations were selfish and vengeful rather than genuinely moral or ideological.
Beyond that, making Midgar sufficiently awful can further reinforce the narrative beyond world building. Because Cloud starts off the game swearing that he doesn't care about the Planet or saving it, and even if that's just his asshole personality talking, the player themselves is equally uninvested in such heroics because they have no reason not to be. So whether the game is organised into a open/world map or just hubs, I'm hoping that there are major points of contrast between Aerith's house and the slums, and Midgar and its wasteland environs and under the sea/the more verdant/pristine locales in the game. FFVII is almost explicitly environmentalist (until they introduced oil in AC/C lmfao), so if there's an overworld, I hope it's as breathtaking and explorable as feasible and that the game actually shows the characters being suitably awed when escaping Midgar or arriving at Cosmo Canyon, especially if it's one of the ones who've ostensibly never much travelled before. In that way, you could increase immersion because both the protagonists and player can justifiably (and simultaneously) evolve from apathy to truly wanting to "save" something as large and as abstract as a planet.
I mean, just in general, the strongest, most resonant aspects of FFVII are the things that are dealt with in passing or only manifest by implication. Like, Jenova is evil and all, but to me the most convincing argument for fighting it, is that what's most worth saving are the people inhabiting the planet, that regardless of what you have seen, they do deserve the opportunity to live. And the most frightening aspect of Shin-Ra isn't that they spend their free time creating super soldiers and beasts, and dabbling in mutation and genetic experimentation. I feel like, thematically, all of that is fluff, or window dressing. It actually might be the least interesting part of the narrative. I'd much rather have a game that emphasises that Shin-Ra is most horrifying not for constructing some terrible monster, but for constructing a society in which a child can grow up deprived of light.