It's a fairly infamous question, and one that has writers give annoyed and unhelpful answers. There are notable exceptions, like this excellent essay by Neil Gaiman.
It's a question I just got asked for the first time, and to my surprise, I realized I have a fairly concrete answer.
"It's not you, it's me!" she whispered, urgently. I dropped to my knees and put my ear to her chest. She was right. The buzzing was coming from her heart, not mine, and it was stronger than I had thought. Not much time left.
Northern Remember to bring a jacket. Take care when entering and exiting the sleds as the station may be icy. Do not feed the reindeer.
Hammersmith and City Operated by dwarves. Mind your head.
Central The last thing you remember is going through the ticket barrier. You're now at your intended destination. Travel times vary widely, but you only rarely arrive before you depart.
Metropolitan Our ongoing modernisation programme aims to eliminate the old entrails-based signalling systems by 2025. Until then, please report any omens to the nearest member of staff.
Ant colonies consist of a number of highly specialized creatures: Sterile females function as workers and warriors. Males only live for a short time and basically amount to a girl-seeking vector for genetic material. And each nest usually has only one fertile female, the queen, who never leaves her chamber and churns out eggs. (There's an exception to this, but I don't want to get into the whole gamergate thing right now.)
So in a classic science fiction approach, let's take a real creature and turn things up to eleven.
In between major releases of Airships, I like to work on things that don't require deep changes to the game's code but make the whole experience better. For version 7.2, I want to make the combat feel more visceral, which of course mainly means one thing: better explosions.
One of my recently discovered webcomics is Stand Still Stay Silent, set in a post-apocalyptic world where a zombiefying disease has wiped out nearly all of humanity - nearly all mammals - except for a remnant in the nordic countries. The disease doesn't do too well in the cold. Iceland closed its borders early enough, and isolated parts of Denmark, Sweden, Norway and Finland survive. By the time of the main narrative, 90 years after the catastrophe, a significant proportion of people now have immunity against the disease, and efforts are underway to slowly reclaim land - a process that requires burnings everything to the ground and letting the cold disinfect things before replanting can begin.
Apocalypses are often an excuse to do away with most of the world and concentrate on some particular part of the world, some way of life that survived, mixing old and new.
So it occurred to me that another interesting way to slice the world would be by elevation instead of temperature.
A common question in science fiction is what humans are like relative to other sentient life-forms. Because we only know life from one planet, Earth, we have insufficient reference points to know how we might compare.
Perhaps the most popular trope that answers this is "humans are average" - we're medium-sized, medium-smart, medium-aggressive, medium-everything. This sometimes gets extended into "humans are flexible", a kind of earthling boosterism wherein we come out on top exactly because we're more well-balanced than the aliens.
I really like to come up with interesting aliens - both sentient ones as well as weird alien ecosystems. One idea that occurred to me a while ago is a planet where all macroscopic animal life is technically the same species.
It's long been fashionable to reinterpret Sherlock Holmes, such as in the recent TV shows Sherlock and Elementary. Apart from changes in time, place, and mood, the exact nature of Holmes and Watson has seen various permutations - here amusingly referenced by Kate Beaton. So here's my cynical take on things: Holmes as a fig leaf for corruption.
Suspendium crystals are what keep the airships aloft. How do they work? I mean, obviously they're fictional, but it's nice to have consistent rules in your fiction. As you'll see, thinking about the rules of Suspendium leads to a whole bunch of worldbuilding as a consequence.
I've been thinking about my Gods & Things concept for Civ V again. I would dearly love to create it, or at least a simplified version, but as it stands, I don't have the time or the knowledge to do so. If anyone has experience with Civ V modding and would like to collaborate, hit me up. Anyway, here are a whole bunch more Horror beliefs:
I always thought it'd be cool to be able to create your own coat of arms in a game. With Airships, this makes a fair amount of sense, as you play as one potential empire in a late-19th-century-esque setting.
This is another installment of "how I would do fantasy creature X". This time: dragons.
I guess the idea of having to make sacrifices to attain magical powers has been on my mind, and this mixed the general weirdness of phantom limb syndrome to produce the following bit of fantasy worldbuilding: You have to lose a hand, or better both, to become a magician.