I can't remember which author said it, nor can I remember the specific quote, but I vaguely remember that it was a female sci-fi/fantasy author, and that the sentiment of the quote stuck in my head as a very good and logical way to consider worldbuilding.
The general gist of the quote stated that it is silly to develop motivations and surface features then try to extrapolate what is happening underneath it all. In much the same way that it is often bad writing to develop the set piece battle at the climax of the novel and then shoehorn all the elements needed to set up that battle throughout the course of the story. When done well, it can be great, it makes elements of narrative like foreshadowing really easy because you know where you have to end up, and what tools will get you there. But it can be a crutch to certain writers, and when done poorly it just looks contrived. Instead create the backstory, and then let the characters and story develop organically from that.
When looking at an RPG, the same things applies. It's often called (rightly or wrongly) "railroading" when a GMs story specifically relies on getting the characters to a certain point that they have set up in advanced rather than just letting the story develop organically based on character motivations. The sense of agency is stripped away from the character and the players feel unfulfilled (I'm getting that a lot from the current Pathfinder game I'm involved in).
So I develop background, I develop interesting settings that will evolve without the presence of the characters through a dynamic tension that cycles through certain concepts. Once the players step in, they have the chance to break these cycles, push destiny in new directions, even if those changes are limited to a small segment of the community.
This whole setting could easily become a "murder-hobo fest", where characters roam through a vast network of tunnels confronting enemy explorers and alien monsters...killing them and looting their stuff. It could just as easily devolve into a purely Warhammer-esque grim-dark setting, almost akin to rent-punk where the characters are barely able to survive, where every victory is hard fought and short lived, and only a very rare few will ever manage to ascend to a life of barely adequate comfort.
Sure, there's going to be elements of murder-hobo, and rent-punk, but these will be balanced against the sense of adventure, mystery and eldritch wonder. There are strange things hidden in the dark places of the hive, some which might not have been explored in years, others decades, centuries or even millennia. There are wars between survivors and shellbrood, between survivors and hiveguard, even between different city-states of survivors. I haven't added these into the maps yet because I'd forgotten about this idea while I was drawing them up yesterday...but I'll make sure to add in a few of these war zones today. That will help determine where specific cultural groups might lie, how those groups differ from the groups they are at war with (and conversely, how allied groups share similarities thus allowing them to interact equitably, and difference that have prevented such groups from simply merging into a single mega-state).
A few other elements distinctly add to the flavour of this setting.
The first derives from the fungal growths that have infested the hive for as long as anyone can remember. The addition of them to the setting was a natural choice due to the lack of natural light throughout the Darkhive tunnels, and the general idea that even the "open" spaces would act like a terrarium with moist humid air. These fungi seem to have been a food source and general resource for the original inhabitants, and the survivors have learnt to adapt to them as well. Some fungi function as construction materials, others as foodstuffs, medicines, poisons or even illicit drugs (thus the introduction of the Panaho race as a specific group who use and manipulate these fungi).
The first fungus on the list is called Manbane, and as the first on the list it is the most prevalent across the hive. This adds another flavouring agent to the setting since Manbane is mutagenic with different effects determined by an individuals chromosomes. Males are weakened by it, females strengthened. I've had discussions with various people regarding this on different forums. Generally, the Manbane kicks into effect at puberty (implying aome sort of hormonal reaction), regenerative powers of either gender are exponentially enhanced (for females) or diminished (for males). Let's say a factor of roughly 13. What it would take a regular female a year to recover from, now takes about a month, and what would take a regular female a fortnight to recover from now takes about a day (vice versa for males). Being out of breath might take a male hours to recover from, while the constant regenerative powers of females might see them never become fatigued. Instead of taking two months, a broken limb might take half a week for a female to heal, and might take years for a male to heal...if they heal at all. Manbane spores drift through many of the tunnels, the spores are microscopic and have limited initial effect except on repeated exposure, but over the course of a few months, the regenerative effects gradually build up. New arrivals to the Darkhive might be able to avoid the effects for a while with hazmat suits and other containment methods, but if they want to interact with the wider community, they'll become infected eventually. The overall effect of the Manbane sees a reversal of gender tropes in this setting, with men as the distinctly weaker gender (often treated as delicate flowers, especially since the fatiguing act of sex might take a male days to recover from). The treatment of males by different cultures will be a distinct defining trait for them.
There is something at the centre of the Darkhive. The exact nature of this thing hasn't been disclosed, I actually haven't thought of what it is and I'm considering leaving it as a mystery. It's dangerous to go down to the inner shells, but people do go down to harvest a rampant fungal weed that neutralises magical/psychic energies. Since I'm thinking that Shellbrood have some kind of hive mind effect, it might make sense that these creatures have subliminal psychic powers, they'd lose their hive mind when in contact with the nega-psychic fungus. So they wouldn't head down to the lower levels, but there would be far more Hiveguard ensuring the integrity of these inner shells (and probably some incredible powerful variants of Hiveguard not seen elsewhere). But if there is a suppression of mystic/psychic powers; we need to see this reflected in the characters, so there will be reality-benders/psychics in the setting (probably predominantly from one race, but this might be a forgotten race who have become so interbred with later arrivals that little is left of them except for their genetic legacy traits in the population).
The last general flavouring mechanism in play comes from the various astral/hyperspace ships that have collided with the Darkhive over the millennia. Few have done so deliberately, more often than not they have accidentally crossed into hyperspace, been caught in the gravity well and have been unable to escape or even send transmissions back to their homelands. There are probably rumours of an astral anomaly that occasionally devours ships, and most civilisations avoid it. The few treasure hunters and knowledge seekers who actively seek out the anomaly are considered rogues and lunatics (but this might explain the prevalence of Khar Tui scholars). Most who arrive, bring strange technologies from their homelands but eventually find their power cells (technological or mystic) run dry.
I'm sure there will be other quirks in the setting, but too many major twists just ends up with a kitchen sink melange that doesn't end up with much substance, I like to make sure my world settings are coherent, consistent, and generally meaningful.