The last time we played in my very-long term campaign setting, the Epic level PCs managed to defeat the 'Cosmic-Horror' Big-Bad by toasting most of the campaign setting, including killing the entire pantheon of Gods and reshaping the planet into an almost unlivable place... but did save the greater multi-verse, so there is that.
The exact reprecussions of their actions were never fully explored, but with our test runs in 5e being a success some players wanted to start the old World up and have some new adventures there... which leaves me with how I should make the campaign setting look/feel after the end.
My first two ideas were:
~World of Islands: land-masses destroyed, leaving only islands (Iceland and smaller) with survivors to rebuild on. The sea is full of horrible monsters, and most of the ocean is either anti-magic zones or wild-magic zones... neither of which make navigation a simple affair.
~World of Tunnels: surface world a wild sea of chaotic-magic, filled with 'you need to be high level to survive here' monsters; and nearly everyone has gone into the Underdark to survive. 'Sun-stone' is an already existing feature in my campaign to make some 'natural terrain' cities underground, but mostly it would be tunnels and occassional forrays into a blasted surface world.
Which would work better for you, or what other cool ideas for an 'after the end' fantasy campaign might be interesting to you?
I like the World of Tunnels better, just because it sounds cooler and because I personally wouldn't want to do a lot of sea-voyaging.
A few other quick thoughts: massive biome type (desert world, ice world - maybe combined with World of Tunnels), or what would Cthulhu do to the place - dimensional tears with fragments of alternate reality located in different geographic pockets.
I'm actually working on a setting right now that is in this general theme.
Basically, civilization was almost completely destroyed in the past by wild magic that was blocked by running water. (The effects didn't stop, though, so monsters, crazy weather, etc, still hit places that were relatively safe). So civilization retreated and started rebuilding on the coasts and islands, with minor settlements by rivers.
Then after a few hundred years of this, people figured out how to make Charter Stones (which I have stolen directly from Garth Nix's Abbhorsen trilogy and not gotten around to renaming yet) that would stabilize magic in the area, and have been slowly rebuilding since. The Charter Stones also severely weaken or block the effects of wild magic, so for example, monsters are weakened in their radius and crazy weather is downgraded to normal weather or blocked at the border entirely.
The islands and scattered parts of the coasts are generally safe, while anywhere more than 50 miles inland is generally a monster-infested wilderness that people are still trying to reclaim. There's ruins and territory just lying around covering the world, waiting to be found and claimed.
There's more stuff, but the general idea is that wild magic is a serious danger anywhere that isn't either fortified against it or by water.
You could work the wild/dead-magic areas between civilizations into Tunnel World. Some tunnel networks are polluted with wild magic, in others magic is dead. the same event could have caused both, or not. These pollution zones could move due to underground air currents or because these clouds are sentient, and they can be sealed off.
The party could come across a ghost town whose fate could be guessed from a nearby leaky seal. Recognizing the signs different civilizations put upon such seals are clues into what lies beyond: monsters, magic disaster cloud, fungal outbreak, plague, poison gas...
Mutations could have come about from chronic exposure to wild magic, maybe forming new races.
If you want to stress the desperation of the post-apocalypse, things like water could be a common measure of currency instead of gold. There could be frequent disease brought on by lack of vitamin D, so there could be sanctified places where the sun shines through cracks in the earth, partially aided by the clever laying of mirrors.
Following the collapse of a civilization, those who first establishes physical safety (ie behind walls) and large sources of food (ie cropland) will likely have a leg-up in society, if they can weather the influx of population. Subterranean cities of gnomes or dwarves might be the richest places in Tunnel World due to never falling, but armed banditry could also increase in lawless areas as it's own form of mafia-rule.
There are likely other lost civilizations older than the apocalypse in deep recesses of the world, too, who think they're the only ones left from whatever earlier apocalypse visited their ancestors. Mind that worked tunnels will look different depending on what civilizations made them, which can be different from what occupies them now. Such considerations remind me of a tabletop game called "How to Host a Dungeon".
Other worlds:
IIRC, Jeff Grubb was working on a campaign setting where everyone settled up in the mountains or on flying ships and plunged beneath the sea of mist to the abandoned lands to salvage old wealth and technologies form the old civilization. They generally preferred to end operations before nightfall, because the valleys really came alive at night. If anyone knows more about it...
I'd say Darksun has been born 
Since magic is running amok you might want to consider not allowing magic using classes for players.
The gods are dead and only the mad use magic.
The world of tunnels sounds fun, kind of like Fallout.
You could also create a world thst used a idea from 5th edition, being that powerfull creatures have a effect on terain around their lairs about a 6 mile radius in most cases.
It might be that these bubbles around the epic monster lars are what keep those parts of the world relativly stable, while the rest turned into a deadly chaotic mess.
Maybe there are tunnels conecting these kind of bubbles.
You could rules that there are more epic creatures that have these effect, or powerfull itemd that do.
For example the palace of a once creat kingdon might stabilise a 6 mile area around the pallice as long as a person of the royal bloodline remains in the pallice.
There would also be the question how fast the changes took place, if it was slow there bubbles could get very crouwded with refugees.
Leading to problems feeding al the peple, and factions that think the population should be reduced draticly to make life easyer for the ones left.
There are several stories with a similar set-up to the Tunnel World idea, namely City of Ember, Breath of Fire V: Dragon Quarter, and the beginning of Gurren Lagann. Even Attack on Titan and Cave Story bear some similarity. They all have some reason people stay holed up, and slowly discovering it is part of the setting's allure.
If your world was racked by wild magic, any number of things could have gone wrong.
- Perhaps danger has already passed. It could be that the drow work to save and organiize humanity during the apocalypse to use them as a serf class. If humanity were to flee to the surface, where drow can't easily live, you can see why the greedy would argue its bad for the drow economy.
- It could be that the sun changed ever since the goddess of the sun turned undead, and her rays cause strange mutations.
- Perhaps a race of monsters on the surface is actively trying to exterminate the "pests" it doesn't know now live beneath its feet.
- Maybe gravity is in fact reversed, and you dig "down" to reach the surface, then probably fall endlessly.
- It might even be the PCs are not inside a planet at all, but a large, honeycombed ship or arc that is still searching for livable land. Maybe the ship wasn't even designed by humans, but by intelligent giants, and humans sneaked aboard to live in tunnels in the woodwork!
A freind and i came up with a "Tunnel World" very similar to that. The surface of the world is a frozen waste and the only heat comes from an incredibly massive and old red dragon living in the mantle of this world wrapped around a core of gold and jewels. What little life that does survive on the surface relies on the scions of this dragon, stealing their hearts and using them for warpmth and light.
So land of tunnels seems to be getting a lot of love... lets see if I can't adapt that to the specifics of my campaign setting...
When the God-Killing entity was finally destroyed (after it killed the primary Nature/Magic diety in the world first, then took out the other Gods afterwards; but in a way that allowed the 6 other Gods to be 'reborn' into 17 new aspects in a pre-planned way); it was attempting to bore into the Elemental Planes and begin unnaming the primal elements (and thus all of physical reality). They 'killed' it by first getting people to worship it, taking a bodyless cosmic horror and (by the rules of this reality) turned it into a powerful but killable Greater God of Destruction.
However, the sudden loss of the established Gods; the total and unplanned loss of the Goddess of Nature/Magic; and thegiant gaping hole left in the world leading to (what we call in this edition) the Elemental Chaos left things in a bad state. None of the new Gods clearly filled the role of regulating either Magic or Nature; and the surface of the world became blasted with Elemental energies. The surface of the world is a giant Anti-Magic cloud, but large 'storms' of Wild Magic energy flow across the surface unpredictably. The storms sometimes come to the surface areas of the underground, so entrances are slowly being sealed off as possible and the trend is to go deeper and deeper to avoid the effects.
The change was quick, but not so quick that people couldn't flee first the the Dwarven kingdoms, then further underground. The surface is now dominated by the two beings that can survive the Elemental energy: Dragons and Giants. However, resources and food are slim, so attempts to raid the underground is very common from these new 'surface' races. Furthermore, unrestrained Elementals commonly manifest in the storms of the surface; and giant universally-hostile Golems also roam the land exterminating all life they can find. The only smaller race to have any significant presence on the surface is Trolls; who were the backbone of the God-Killer's armies.
Dwarves are ironically now the race associated with the surface, their kingdoms being nearest the surface and turned to guard the entrances to the surface world. They occassionally make exploratory trips to try and recover and scavage what they can.
Elves are secluded in their magical cities, which use Sun-Stone to grow forests and plains and the like to simulate their old lives on the surface in intentional denial of the world's problems.
Gnomes have moved in with their svirfneblin cousins, and isolated themselves in caves with few entrances.
Humans and Orcs are mostly slave races to Drow, Illithid, and Aboleth; all of whom have boomed into significant 'Golden Ages' and fight mostly eachother for dominance of the old Underdark.
Halflings have all but disappeared, living only under the protection in small enclaves of Dwarf and Elf protection.
Goblinoids are everywhere; both as savages and hostiles but also (out of necessity) trading partners and defensive-pact allies of the civilized races.
Adventures at low levels will be simple matters of survival, escalating into a combo of dealing with the vestiges of the Cult of the God-Killer and searching for a way to make the surface habitable again as Epic levels approach.
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Sound like a good place to start?
That's fine if you don't want gnomes and halflings to play much of a role in this new world. Why would anyone interact with the elves, for that matter, either? Tho' isolated, maybe most food or sun stones come from their realms. Maybe the gnomes are building downwards in an attempt to escape, but they "dig too deep" and find some horrible secret about the world far beneath. Maybe the halflings are endangered species, but one is somehow needed to fix things. Maybe some led an exodus of others into another plane and nobody's heard anything since. Of course, when an apocalypse shakes up the racial niches, racial relationships can change, once obscure races can become player races, and once common races can be extinct. Even racial traits could change due to curses and mutations.
A big part of the campaign is going to be the racial and national divisions that surface after the 'great alliance' against the God-Killer stop holding everything together. There will be adventures and background maneuvering about getting the races to cooperate again rather than the isolationist policy many have adopted. The main goal of the cult the PCs will be aligned against will be to ressurect the God-Slayer not because they like it but because giving the world an enemy to stand against prevents petty squabbles.
Specifically to gnomes and halflings: gnomes were always somewhat of a 'minority race' in the setting already; and halflings I'm sort of offering up a 'sacrifice' to show how bad things have gotten.