Before I started writing my newest novel "Hero of Fire", I took a few weeks to build a world where Djinn live. I first researched all the classical sources on Djinn including: Arabian Nights, the Talmud, and the Koran. Piers Anthony did a great piece called "Hasan" that I also re-read. I mixed in several non-ficition pieces about the modern day Middle East. There were several odd, unexplained passages in several of the prime source materials that I decided to leverage in with my world.
I decide to predicate everything on one wild theory: Djinn are crystaline-based intelligences, rather than organic carbon. This makes them almost immortal as long as their core-stone isn't accidentally shattered. It also explains why a Sahir with a ruby or fire-opal ring controls the Djinn servant--if he possesses the core, he can threaten to destroy the Djinn.
Then I take this assumption to natural extensions and fit it into the known lore.
+ They must keep bargains because they are written into the information in their core. Any Djinn children would have a copy of this information, making covenants and grudges generational.
+ There can be Djinn analogues to almost every form of life: bacteria, plants, insects, fish, birds, mammals, etc.
+ They generally have more inborn information/languages/historical knowledge than humans but don't bother with reading or school.
Let's start with one Djinn tribe and show how we can rewrite the rules behind the scenes using this secret key. I begin with the most shunned and "evil" tribe--the Ghul (or ghouls). The legends say they don't change forms, but appear as human males. Ghul lurk/dig in graveyards or ambush lone travelers for a snack. Other Djinn disdain them.
The angle: remember the crystal lump from Harry Potter that could cure any poison: the Persian bezoar? Goats and even prehistoric animals had them riding inside. What if the bezoar were the Ghul? What if it takes over the bodies of recently the dead? If someone dying of poison took one, the bezoar could miraculously "heal" him by having him possessed. If their host is too damaged, they need to dig up or "find" a new corpse to inhabit. This technique for interacting would explain the animosity most tribes feel for the Ghul. Those with religious objections or insufficient skill could take over animals. This subtribe is known as the Hinn and would explain why Solomon could speak to birds and use them as spies. It also explains odd passages in the Talmud where a bird or snake touching an unusual diamond becomes intelligent and carries it away.
Does history fit? Add in fears of being buried alive, zombie tales, vampire stories, and Elvis sightings and we have a consistent framework. The health craze in the 1800s of grinding bezoars to dust would have driven them to near extinction. Guns were a horrible invention because they could decimate a host and possibly shatter the stone by accident. Plagues were a good business for the Ghul. I could see Napoleon's return from Elba being a Ghul--he was poisoned a lot. Their culture probably had to adopt rules about taking over famous people and leaving evidence.
Then I pick a concrete Ghul character and tell history from his point of view. My female lead became the former spy Mata Hari. Researching the woman, her life changes radically from a Java military housewife to a stripper with a made up past--shortly after her family was poisoned by a rival of her husband. In the 1950s, her head disappeared from the museum--because it was medical evidence. Reborn in that era, she would have taken over someone with the face of a starlet; however, she'd still have the same French accent and her eyes would seem similar in photographs.
I can easily spend 30 chapters viewing the world through this skewed lens and challenging my male main character.