Your tastes may differ but I really love High-Fantasy, High-Magic settings.
But usually, every time I try to create a homebrew world, I go too far and add too many oddities and then I stretch credibility too much. There is a thin line between originality and ludiscrousness.
I'll try to be more cautious. But not too cautious.
Zeroth Theme: Thematic Unity
A good setting should have a limited number of assumptions which can be summed up.
First theme: Immanent Gods
For instance, it is a standard assumption in fiction and game settings that Gods should not act too much within the worlds. Else, you would have a dilemma: either they overpower the PCs or they are not divine anymore.
But I prefer that final risk. I want mythic Gods who try to seduce mortals and can be afraid of mortals. But I don't want them to be simple "Superheroes" and they still need to keep some measures of AWE.
OK, so the Gods are present, they mingle with worldly affairs but they are still hard to understand and have contradictory aspects. They are not on another Plane. They live in Temples and their mansions.
Second theme: Reincarnation & Remembrance
There should not be too many sentient species. D&D worlds have too many of them.
But on the other hand, diversity is fun.
I really like the Hârn solution: there are only a few races but one of them (the Ivashu) has no stable forms and they can look like hundreds of different species.
Tthere are only two major sentient species. Let's call them: Mortal Humans and the Unseen (Spirits/Faeries/Djinnis - I may add Dragons).
The Unseen assume many shapes since they are reincarnated and - contrarily to most Mortal Humans- can remember some parts of their identity. Some are reincarnated in sentient animals (Cats for instance), other have created improved bodies (and they are the various Elfs & Dwarfs), or elements (Elementals).
Third Theme: Promethean Art
But...Magic is an Art whose essential purpose is not to control Nature but create new Life. Therefore Magic-Users have created various creatures and given life to their works.
One of the first Arts was Daedalian Symbolic Sculpture: the art of creating Living Statues, Golems and Colosses. The Unseen love to give birth to this Spark of life but there is a controversy: do they really create new Life as they claim or do they only catch and seal some oblivious Spirits of parts of the World-Soul into their works?
Elfs are the Faerie upper castes who love delicate talking Gems, or precious Statuettes. Dwarfs are Faerie proletarians who created the Stone Giants.
Fourth Theme: Mortality and the After-life
The Mortals know they can die and are not sure they can be reincarnated (or if they do, they could become slaves to the Unseen). One of the major goal of the Mortal cultures is to find out what happens after death and if they could reach some kind of Immortality like their Faerie counterparts.
Fortunately, there are many Gates to the Otherworld.
One of them, Limen the Twilight City, the Obsidian Gate, is on the threshold of the World of the Dead. The city has a third category of being between Mortality and Reincarnation: Ghosts.
Ghosts remember some parts of their previous lives, like the Unseen People. In fact, some believe that the Unseen were some kinds of Elder Ghosts who stayed on this world.
The Psychopomp has his ship on the Sunset Shore of Limen and he takes periodically those waiting souls to the Hereafter. Some Mortals tried to attack his Black Ship to conquer Death itself. People say they now row on the Black Galleys.
Another city, Relic, claims to have a limited supply of Eternal Life Philtre. This is controlled by the Immortal Council, who share the flesh of the "Sacrifical Host". Some say it was given by God or that it is the Flesh that God left, as a snake sheds his skin. This Divine Manna or Soma is actually the Flesh of the Immortal Behemoth (I stole that from the City Built Around the Tarasque from RPG.net). It can cure wounds and stop aging but if you eat some bad parts, you could become an abomination, a cannibalistic Ogre or half-zombie Ghoul, a regenerating degenerate Troll or a Vampire.
Alchemists look for the four humors of the Behemot: its Ichor, its Phlegm, its Melancholy and its Yellow Bile.
The Great Immortality Wars were caused by many factions who wanted to control the Heart of the Host, the only true Elixir of Immortality. But people are afraid to attack the Holy Relic for they believe that the Behemot could awaken and be free. A Prophecy even said that it will happen at the End of the World, "when the Bell tolls".
There are some strange beings which come from this unearthly unliving corpse: Immortal worms, Immortal Flies and other strange plagues (including the Immortal Moss which grows on the Undead Armada).
- Other Cities
Eclipse is a city which was built under the Moon. It is always in the dark and the Moon stops the perpetual Starfall which built the Crater of Eclipse. Eclipse is the city of the nocturnal Vampires, of the were-beings and of Mages. One of the Unseen tribe took the shape of Shades. They can also travel in Dreams.
Electrum was one of the famous Gem-Cities. Some of them vanished or were eaten by Dragons. The City of Electrum was reduced in size, to an Amber crystal, and its Dwarfs are now tiny like insects. They built Metal Figures who wage war for them. They also shrank whole mountains of Rune-covered pillars which are supposed to be the largest Library. Beware of pebbles which could be a whole Babel Book. Some say the Electrum Coin was split in two parts and that they are reunited the City of Electrum will come back to its original size.
A story says the Behemot (whose body is kept in Relic) was not unique. There is another living creature beneath the sea and there is even another city built in the inside of the living Leviathan. Some say there are sentient and malevolent Krakens who rule this Leviathan. Other say they want to free the Behemot or that there is a secret society which wants to poison the Leviathan. Their city appears periodically as an Island and they own the Bell which will tell the day of the return of the Behemot.