Origin of the World: Most societies throughout the world and history do not separate the origins of the natural world from the origins of the human world. Every society views itself as the normal, natural way for humans to live, and origin stories reflect this view.
A highly religious society is going to have a highly religious-centered origin story, filled with higher powers that deliberately create the natural and human worlds for specific reasons and set down the social values viewed as most important.
A highly secular society may either be indifferent about "where it all came from," assuming things have just always been as they are (perhaps in your fictional world this is true), or they may be searching for the truth about where the world came from.
Origin stories also usually set up the basic social order that the modern society still follows, and both the creation of the natural world and the creation of the social world tend to reflect a society's over-arching social order and power structure. A society where men have all the power, for example, will assume that men invented everything and that the natural world was created by either masculine higher powers or through traditionally masculine activities - hunting, war, sex, competition, and physical labor. The opposite type of society might have origin stories that include traditionally feminine activities - giving birth, teaching, bargaining, some form of artistic activity or dreaming.
On Non-Human Races: Biologically speaking, it is more common for there to be more than one variation on a species alive at any one time. The fact that there is currently only one variety of human in our world, for example, is a rare exception.
Social Institutions, Groups, and Player Character Factions:
Role playing games usually include certain factions or groups that players must choose to be affiliated with, from character classes to clans, tribes or nations of origin. If you want to make these factions believable in your world, they cannot exist in a historical vacuum. They have to have come from somewhere.
The First Law of Change: In the real world, every larger social entity - government, religion, clans, aristocratic families, corporations, etc - experiences some change over time. There is no such thing as the "changeless, immutable holy order that has lasted for thousands of years." That may be what they believe, but if this so-called unchanging order were to hop in a time machine, they'd be surprised by how they looked in the beginning.
The Second Law of Change: Society changes only when something prompts it to change. Not necessarily a crisis, but Earth's history teaches us that any major social change inevitably generates a point of crisis before society fully adapts to the change. Major catalysts for change include:
- War: World War 2 and women's liberation in the United States
- Religion/Philosophy/Ideology: The invasion of Jerusalem alternatively by Christians and Muslims during the Middle Ages
- Environmental or Climate Change: The changes wrought in Europe by the Dark Ages and the Black Death
- Deliberate Invention or Innovation: Thomas Edison and the electric light
- Accidental Invention or Innovation: The practices associated with most modern Western holidays, from Christmas trees to Jack O' Lanterns.
The Fourth Law of Change: Groups of people only adopt an idea, practice, or piece of technology if it meets a need better than what they already have available. The major needs are:
- Survival: Food, water, medicine, shelter, companionship
- Sex: Access to potential or specific partners, being seen as more desirable/sexy/beautiful
- Status: Something that is believed will improve one's social standing within a group, make one famous, make one powerful
- Profit: Something is seen as a business opportunity, a chance to become rich, a chance to break out of poverty.
- Social Justice: Something is adopted because it gives a chance to change social systems to better suit a specific group within a larger social order (ex: empowering a minority group).
Origin of Powers: Most role-playing games include supernatural or extraordinary powers, abilities, or technology.
Definition: "Supernatural," "extraordinary," or "special" all describe something vastly unique and outside what is normal in a given group or world. If your game takes place in a world of psychics, then being a psychic is NOT a supernatural power. Any more than being a musical prodigy is.
In most games, powers have formal systems of groups and sometimes formal institutions to back them, such as a wizard's school or a super hero team. Everyone in the world refers to these powers the same way, groups them along the same lines, and recognizes others who share special powers by looking for certain markers - symbols, costumes, etc. This is called standardization. Here is why it happens:
The more complex a society becomes, the more specialized knowledge gets. In order for an individual to become fully trained in a specialized body of knowledge, they usually have to be isolated in some way in order to focus time and thought on learning and practice. Also, everyday language quickly becomes inadequate for talking about a specialized body of knowledge between specialists. Hence why every role-playing game has a specialized jargon that is used in-character.