Some game rulebooks provide core assumtipns for the world. I usually print these and put them on my GM screen to help me maintain a consistent tone.
For example, the Forgotten Realms in D&D has the following.
• Gods Oversee the World
• Much of the World Is Untamed
• The World Is Ancient
• Conflict Shapes the World’s History
• The World Is Magical
The Primeval Thule campaign world (heavily inspired by the writings of Howard, Leiber, Burroughs, and Smith) has the following.
• Thule is Barbaric
• The Wilderness is Savage
• Cities are Wicked Places
• The World is Mysterious
• Magic is a Secret Man was Not Meant to Know
• Ancient Evils Threaten Mankind
• Freebooters, Mercenaries, Opportunists
What do you think would be core assumptions of Howard’s Hyboria?
I’m thinking that the ones the Primeval Thule authors use are a good fit, though I might also add “Humans are newcomers” to the list.