I can't even imagine what this would look like! Sounds cool, though.
The world is divided into Eight single-biome (more or less) Worlds, including the Underdark and Star Heaven (which are also always below/above all of the other six), that are connected by the ancient and mysterious Warp Pipes. Plains World, Jungle World, Desert World, Ice World.
The playable ancestries in D&D terms are Humans, Tortles, Gorilla-Orcs, AD&D Kobolds/Halflings, venomous Vegepygmies, and CN magical rogue Warforged. Room for more, certainly: Hobgoblin/Oni, Thri-Kreen,
maybe Gith would all fit. The Gorilla-Orcs are roving bandits and pirates, while the Kobold-Halflings prey on the cities. Vegepygmies are used as slaves by more powerful civilizations (all of them). The Warforged are
terrors; they're mercenaries, disciplined and professional on the job, who spend all of their blood money on supporting scholarship and the arts, and performing elaborate random acts of senseless violence/kindness.
All of the Worlds work on a
Points of Light framework. Human cultures are either barbarians, scrappy pastoral folk, or decadent city-dwellers. Tortles are in a brutal civil war between the Loyalists serving the (centuries) new Evil Emperor and the Traditionalists trying to preserve their ancient LN(G) dwarf-druid traditions.
There are no horses. People who can afford them ride giant flightless birds. Even wealthier people can afford giant
flying birds. Magically talented craftsmen can build automobiles and skyships. The most common weapons are hammers and spears.
Everything is magical, and powerful PCs/NPCs are explicitly magical regardless of class.