I really enjoy worldbuilding, and have a list of ideas from the mythical and fanciful up through grim. My average campaign runs 4-7 years, and by the end of it I have only once set a new campaign in the same world. In that case it was 80 years later to allow what the PCs did to spread and have ripple effects, and to refresh problems, NPCs, and the dynamics of what was happening. I have more settings I've already thought up then I will ever have a chance to run.
My current homebrew setting is for my Masks of the Imperium campaign. All of the PCs are Mask Bearers - the 101 Masks are ancient intelligent artifacts created at the peak of the long-failing Imperium*, each that awakes only once every few generations as it finds a compatible bearer, and they take years or decades to fully awake, slowly granting more powers. The characters having authority from level 1 made low level adventuring much different than the usual fetch quests and goblin hunting, and that they all have legacy magic items that grow with them plus they can requisition any mundane equipment has made it the least murderhobo campaign I've every run.
* According to the Histories, but the players have found that the Histories have definitely been written by the winner, and often contain propaganda. The only who can gainsay this are the long-lived elves, and they have slowly become 2nd class citizens who have been "granted" reservations where they can live separate from the other races.
The world itself is the body of a dead god(dess), and all magic emanates from it. Her skull circles as the only moon - though most people think that's a legend because "yeah the moon does look a little skull like from the right angles".
The literal Bones of the Earth contained the highest concentrations of magic in the body, and the Dwarves were hording it**, so the Imperium for the good of all wiped them out and created the Drow (non-Lolth connected) to live in their underground spaces and mine it for the good of all.
**Read: The High-Magic Imperium coveted it, genocided the dwarves, and made a race to that couldn't stand the light of the sun to go into the dwarven strongholds and mines and get it for the Imperium.
The Imperium also created Halflings, a gentle race of agricultural servitors. (And also messed with the human noble bloodlines, which is where variant Humans come from.)
But the Imperium has been failing for a long time, a fraction of it's size. The Child-Empress Olixia, IV of Her Name, took such a concentration of Masks awakening at the same time as an Omen, and kept them together, sending them to a colony in a newly discovered land across the magic damping waters. It seems that once you get far enough from land, magic doesn't work, and the high-magic Imperium has used magical methods of navigation and propulsion for centuries. So the continent and various nearby islands and archipelagoes were known, but not this place.
Arriving at the new continent, they find magic much more plentify. Lost Great Spells of the 7th and higher circles of casting would probably function here! This is where the PCs started to realize that where they came from was very low on magic, a good reason while the wonders from the peak of the Imperium like the sky-trams no longer crossed the continent. It also contains high-magic creatures that no-longer can live near the Imperium, like Unicorns and Dragons.
To get back the the setting, the Imperium is vaguely Roman in description, including titles and such. There are known portal to the Feywild, most that can be unlocked and opened at particular times or with certain objects, songs, etc, though there is a huge permanent one in the center of the largest Elven reservation. (Which the players managed to mess up, but that's not setting, that's campaign.) Time is wonky between the feywild and the material, often at the whim of Archfey who control the various portals.
There are also portal to the Fiendish Lands, guarded by a order of Knights of the Sacred Flame (player created, otherwise I wouldn't have used the a name the same as Eberron uses). It was founded many generations back when the heir to the Scepter (symbol of Imperial power, not a throne or a crown) was seduced by fiends and a great champion was able to slay him as he attempted to summon fiendish hordes. But the champion was guilty of regicide, which accepted no excuse. So an order of knights you could join that were you could leave behind your whole life and take another, dedicated to nothing but guarding the portals, was born. (This grew from the background of one of the PCs.)
The new continent has a whole bunch of unusual combinations of races on it. I specifically wanted it to defy expectations. Like there are your "standard" giants in the Imperium, and they and the elves have warred for ages. But there are peaceful agrarian Frost Giants in the new world. On the other hand there's a large, extremely community oriented, kingdom of xenophobic forest gnomes who humanely collect blood as payment for any crimes or other transgressions against the community which the feed to either their leader or their god, it hasn't come out in play which. Their leading warrior caste are were-leopards, another point the party has yet to find out.
The Masks themselves have a lot of history behind them, and in case my players come by I'll leave it to say they are more than they appear.
Okay, that got longer than I thought, and it just scratched the surface. Next session is #35 and lots of things have been fleshed out.
