Still don't have a fully fleshed out game design outline, but I do have civilization concepts for a hypothetical fantasy RTS.
The white-skinned, golden-haired Norvolk eke out a harsh existence as hunter-gatherers in the world's northern steppes and boreal forests. They do not build permanent settlements, only temporary camps of leather-and-bone tents which they can dismantle and move when need be. They have democratic systems of tribal government in which chieftains are elected by popular vote; these are usually the most highly respected hunters and warriors. Their religion is animistic, featuring the worship of spirits believed to permeate the natural world. Despite their generally primitive technology, the Norvolk are skilled metallurgists and formidably powerful hand-to-hand fighters, though they frown upon the use of ranged weapons as cowardly. They usually travel on foot although higher-ranking tribesmen may ride horses.
The Taridim are an olive-skinned, hook-nosed race who live in hot deserts. They used to be an assortment of squabbling nomads like the Norvolk, but ever since they were united under one banner by the charismatic prophet and warlord Yaghoub, they now cooperate as a tribal confederation under the leadership of a priestly council. The Taridim have matured into a thriving civilization ever since their confederation's birth, growing prosperous off trade and farming alongside the desert's scattered oases. The Tarid religion is polytheistic, with its gods appearing as multi-armed humanoids with unnatural skin colors, although the most important deity is the moon god Azkar (who is believed to have created the world). The Taridim are particularly proficient at archery and riding camels and elephants into battle.
The black-skinned, woolly-haired Kembweans reside in the tropics and have the world's most ancient civilization. They are ruled by a hereditary monarch who may either be a man or a woman and is believed to be descended from the sun goddess Khamunra. The Kembweans are renown around the world for their sophisticated, monumental stone architecture; their temples and royal tombs are the most famous, but they are also excellent at constructing durable fortresses. They also have advanced mathematics, medicine, and beautiful sculpture. Although the Kembweans are polytheistic, most of the clans that make up their society concentrate their spiritual energies on a relatively small number of totemic animal deities; veneration of ancestral spirits also features prominently in the religion. The Kembweans have tamed a large variety of large animals (e.g. elephants, rhinoceroses, and zebras) and so have among the world's best cavalry.
The yellow-complexioned, slant-eyed Shangchi reside on an archipelago of islands with a temperate climate. They once had a mighty empire covering the entire archipelago, but have fractured into many lesser kingdoms that chronically war against each other. What the Shangchi share are an atheistic philosophy and an affinity for technological development. They are the only race in the world to have gunpowder technology, although it is very costly to produce, and their warships are the largest in the world.