d20 Fantastic Skies

In real life, the sky isn't really a thing.

pictured- a clammy gulf of nuthin and things too far to reach

How about in your game?
Sure, weather is a thing. Clouds get some love. Maybe your sun and moon are allegedly gods, but do they act like it, or do they act like distant balls of rock and/or fire made of physics?

pictured- an interpretation of the sky that's a lot more gameable than 'clammy empty gulf'

 

So here's some fantastic skies that might actually be relevant to actual gameplay. Some are even compatible with each other.

1. The sky is a giant rock, just like the earth. Clouds and moons and stuff float between the sky and the ground, and might collide with tall or flying objects. The highest mountain in any given region is sacred, because it holds up the sky and stops it from falling and crushing that region. In mountain-poor regions, trees have to bear the burden of the sky. Either way, you can reach the sky and mine it, looking for stars (glimmering gemstones) or star-metal (regular sky stuff is just rock colored like the sky and it looks pretty lame up close, you gotta get the good stuff).
2. The sky is like a giant tarp, a flexible membrane. If you cut it open, stuff escapes into the Void in catastrophic decompression hurricanes. Eventually the hole seals. Sometimes things from the Void get IN though. If you get sucked into the void, you'll find the outer shell of the sky is grizzled and tough and much harder to break through, and littered with debris from past punctures. And also Void stuff, which you don't need to see much of to be glad it's in the Void and you're under The Sky. "Splitting the sky asunder and ushering in the Void" is every other doomsday villain's plan.
3. The night sky is Nut. Nut is that there egyptian goddess featured above. What she does is tiptoes around on fingers and toes, careful not to crush things, round and round the world, bringing a respite from the sun.  Stars are her jewelry, and the King of Thieves is coming out of retirement one last time and putting together a team to steal her nipple piercing, and there's just one question you should be asking yourself right now- Are you in?
4. There is no sky- blue is just what light looks like when bounced through the atmosphere, and stars are just other towns seen on the far side of the world, because you're on the inside of a sphere, not the outside. Legendary archers can shoot all the way across, and the Ancients built a tower that spans the gap, now a forsaken ruin squatted in by pretender kings and forgotten horrors.
5. Each day's sky is a different giant whale- typically blue whales, but there's a lot of whale species you know, one for each type of sky, except night skies. Each night, the whale dies, rots, turns weird bruise colors(the sunset) then turns black (Night), and is devoured by the Star Brood. Stars are just huge glowing maggots. Sometimes they fall out of the whale and cause trouble. Sometimes hunting the sky-leviathan for its incomparable bones and flesh gets the whales before the maggots do, but it will require a certain ship, and a certain expertise, and a certain harpoon
6. Stars are all suns and also sons, but they're banished from the court of the Celestial Sun Emperor. Every year or so they try to stage a coup and earth becomes eternally day and way too hot (this is known as War Summer), so the Sun Guard have to assemble to slay or drive off the sun or sunS. Some disgruntled rebels even side with the would-be usurpers and the Sun Empire is wracked by civil war, while those less honorable still take advantage of the disruption in social order via banditry.
7. The sky is the ocean, obviously. The ocean is blue during the day, like the sky. The ocean is black during the night, like the sky. This ain't rocket alchemy. As such you can sail through the sky just as you can sail through the ocean, though since it's upside down, only dead people can stand it for long periods of time. Stars are the boats of the dead and their corpse lanterns, and the sun shines on the Dead Ocean and the Live Ocean equally.

   

   and cloud krakens mostly eat people on the Dead Ocean but don't you trust fog banks

   
   

   
   

8. The sky is a facade concealing the TRUE SUN from the world, set there by the FALSE SUN. The TRUE SUN's light shines through holes in the facade, so brilliant that the light of the FALSE SUN looks like darkness in comparison. The MOON is the FALSE SUN when seen in the light of the TRUE SUN shining through the STARS, which are holes shot by arrows of heroes who tried to fight the FALSE SUN, but the cowardly FALSE SUN ran away and so the arrows struck the facade instead.
9. There are a seven or so skies, each a different color, layered atop each other like a rainbow. Currently, Sky Blue and Sky Black own about half of the sky, and take turns shuffling over. The sunset are the lesser skies revealing themselves in the shuffle. The ocean and land were once skies that retired to lounge around the ground, and sometimes are visited by the lesser skies. To land and ground dwellers, this is like another dimension overriding their familiar landmarks, and so there are 9 worlds, one for each sky color and also land and sea, layered and overlapped.
10. The clouds were the color of a hooker's bruised eye, and the rain beat down like the john's fists responsible. The streetlamps bled red into the gutters next to the American dream. The dame who had walked into my office was a rosy-cheeked thing called Dawn, and she was looking for a golden ball, and after asking all the birds of the air, the fish of the sea, and the worms of the earth, it was my turn to ask a few questions. Who hired these stormclouds? What's a east-rising Dawn doing with a West-side private dick like me? And most importantly, where the hell had the sun gone?
    I had a feeling before the end of this, I'd be sorry I asked.
11. The sky is the chariot-track of the gods, and the race and the bets occur every night. You can make a fortune betting on what star will rise or set first, but with a thousand thousand stars and a hundred hundred astrologers, there's no such thing as a sure front runner. Get yourself a gleaming steed and you can even race yourself, but there's a catch- the gods can survive the coming of the Sun but any mortals laboring along the track will burn come dawn.
12. The sky is a lens over the City and the Desert, through which the great and terrible One-Burning-Day glares and judges individual deeds, and the cold and merciless Trillion-Eyed-Night judges the context of those deeds. Those who do something out of character are cursed by Day, and those who do something in violation of cultural norms are cursed by Night. Only those who hide indoors and underground are free from judgement, and so it is only the nobles in their palaces who know only the light of Flame who may do as they please. Those who wear no clothing have nothing to hide, but those swathed in garments are surely trying to evade judgement. Some people claim that if you reach the horizon you will find the glass of the Lens blocking escape, but others say if you can escape the Lens, so too will you escape the twin tyrants of Night and Day.
13. The sky is a book, and every day, a new page. The deeds of the day are written in the sky until it is black with ink by a scribe, and then the page is turned to begin anew. The wise can read the sky to determine what happened or even is happening in distant lands. Some pages have been stolen from the library of the scribe, and a single page contains more knowledge than a human mind could learn in a thousand years, though it describes but a single day.
    If you destroy a page, everything of that day will be forgotten.Will you brave the library to kill the past, or to remember what only the scribe knows? Either way, the scribe could use a new roc-feather quill...
14. The sky is the world to come, and the world that was. Clouds are prototype animals, land-masses. You can catch them and they become real, or rather they were always real, but they gain substance and detail. At the Edge of the World, Horizon, the clouds become new lands and frontiers, and old, worn out lands and animals become clouds. Dead souls become clouds, and you can visit them and they you, all vague and white and fluffy. However, there is a balance- for every cloud given substance, some substance must lose itself to the half-imagined cloud realms. 
15. The sky is the dream-realm. That's why humans so often fly in their dreams you see, and daydreaming is sometimes called having your head in the clouds. It is also the nightmare realm, as so often humans fall in their dreams and wake up- that's their souls falling back into their bodies. Your limbs feel heavy when you're tired because you long to return to the weightless dream-realm. Your dreamself is usually quite similar to your waking self, but idealized. But as you ascend past the sunset cloudlands of dreams and into the dark heights of nightmare, problems occur- missing pants, slow-footedness, teeth falling out. The dream world copies the real world in golden clouds and blueness, and high nightmare in blinding whites and shades of darkness. Dreamers can retrieve items from the sky- gluttons retrieving remembered pies, adventurers dreaming of failed dungeon heists turned successful- but these items are accursed, causing insomnia, amnesia, madness, and drawing attention from the dreamrealm even in the waking, or the thieves being denied access to further sleep by screaming storms and lightning.
16.  The sky is much like the sea, which is much like the land. The land has kings, the sea, sea-kings, and so the sky, sky-kings. Sure, the sea-people are fishy and the sky-people are birdy, but it's all the same really, just another strange far-off country, a palette-swap parallel. Concern arises of course when a realm of land, sea, or sky unites to war against another realm, for princesses or honor or whatever foolish notions cause men of sea or land or sky to war.
17. The sky is forbidden. People wear huge-brimmed hats that droop down to obscure the horizon, aware of each other mainly by their feet.  Sun and stars and rain and shine all fall equally upon the wide wide hats. Of course some people have looked, been punished. They tell you they wish they hadn't and they seem sincere in their downcast way. Sometimes there are strange lights and noises from above, and staircases to thin air that would take one above the hat's power to hide the sky above. But forbidden, always forbidden.
18. The sky is fire, a blue fire, and a distant black field of ash. The fire comes and goes, and like fire, it puffs smoke that we call clouds, and melts aether into rain, and has glowing embers called Sun and Stars. Sometimes, you can get a hold of sun-shards and star-metal and make something of the True Flame, but sometimes, a cold greedy animal of our cold, greedy world eats True Flame and turns into a dragon. And once something has the True Flame inside it, only the True Flame can stand against it- our shadow world, our ash world, has but false red flame that's nothing compared to the blue.
19. The sky used to be all white, until the gods hired nymphs to paint it. The nymphs started painting the whole thing blue, until the gods complained it wasn't different enough from before, so the nymphs started painting the other half black because black is the most different from white, yeah? The gods realized these nymphs, despite being beautiful themselves, were not good at making other things beautiful and called them off, leaving small specks of unpainted white in the black side of the sky. Now mortal artists are called in to paint the sky, but there's so few of them and they can only paint a small portion each sunset and sunrise, which gets lost in the tide of blue and black paint the nymphs left behind. Water and Darkness, the Blue and Black, the paint that dripped down, is collected and used to paint things into reality, provided they be black and blue alone. Collect nymph hair for brush, and you too can try your hand at Painting.
20. The world is a bubble adrift in primordial chaos, and the sky is a great wall constructed by the pantokrator to keep chaos out, and you approach it by walking clockwise, and leave it behind by walking counterclockwise, adding a few extra dimensions to dungeon castle design and travel planning. Metaspatial geometry aside, leading theologians have discovered that contrary to previous thought, we're on the chaos side of the wall, but no one is sure if humanity is the equivalent of peasants working the fields outside their lords stone castle, or if we are the barbarians at the gate of law. Investigating the blue castle that is the sky and its hordes of defenders, visible as stars in the night, will be the only way to know for sure.

4:06AM plz kill me