Things we decided on on IRC:
tl;dr:
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• We'll start with a small region, flesh it out, and expand from there.
• The setting will be realistic and subject to the laws of Earth.
• It can still serve as the setting for any kind of fantasy. Canonically, these stories would be instances of historic inaccuracy, or folk tales, centuries later refuted by modern science.
• Think of the environment and its inhabitants as Earth+. Life is continuous with Earth's, but might have evolved into curious shapes.
• If something is added to the setting, it suffices to have a plausible explanation for how it came about, details gladly omitted.
• The foundation will lay a map, a timeline, and a collection of scattered ideas.
We are going to start with a geographical area of modest size, host to a good handful of polities and regions. Should this work well, we could gradually expand the world—reveal more of the greater whole the region was part of, elaborating on brief references to other places and peoples we might have dropped.
The question of whether to make the setting fantastical (magic, mythical creatures, etc.) or realistic is resolved by considering any works and writings of us not as words of god defining canon, but as part of the universe.
A heroic tale of dragons and their slayers would have its roots earlier in the timeline, and the creatures and phenomena it describes would—come a few centuries—be debunked and explained by modern science. The devil king was what survived of a particularly cruel ruler in collective memory; the betentacled demon resting under the mountain an interpretation of encounters with a rare species of blind snake which lives in caves around the area.
Any works of fantasy set in the universe would be recognised as such in the canon. This allows us to use the setting from anything from high fantasy over modern thrillers to science fiction without becoming inconsistent.
The fauna and flora are going to be continuous with Earth's. Think of the world as Earth+, with a continent having been inserted somewhere in our world. Various real species thrive there, but others might have undergone curious evolutionary processes over millions of years, and, sharing only distant common ancestors with them, might deviate greatly from creatures we know. Think of it as alternate evolutionary history.
This will allow for creative freedom, but avoid veering too deeply into fantasy territory. There might be reptiles, having grown to great size due to island gigantism for example, who spit stomach acid or venom, but not something as unrealistic as flying, fire-spewing dragons—although the people thousand years ago might have seen it as such!
As humans are subject to evolutionary forces just like any other living being, fictional races could be the substitute for things like elves, orcs, or dwarves.
Maybe a people settled on an island where a lethal parasite resided; it costed the lives of many, and only a few individuals with unusual mutations allowing them to enter a symbiotic relationship with the parasite survived. This founder population came to shape the peoples of that land, though the parasite deforms them horribly, or affects some functions of their body—possibly to their advantage! Any invaders fell prey to the infection, and thus they managed to stay around for a very long time.
Elsewhere, archaic hominids—brutish, simple, bulky—might have survived isolated within inaccessible mountain ranges. Again, alternate evolutionary history.
I am going to try and create a map which we could fill with cultures, creatures, and ideas. The framework for our works will be a timeline, outlining briefly major eras and happenings in the region's history. It would then be up to anyone to elaborate on any of them, to pick any as a background for their works, or to fill the many voids in between.
Let's use this thread to collect ideas! Creatures you would like to see roaming the lands, nations you envision, anything!