Try as I might, I haven’t been able to track down where the name suli came from. The best I’ve come up with is Abu Bakr al-Suli, a tenth-century poet and chess master in the Abbasid court. In this context, adding al-Suli to his name is like saying “Tiberius the Claudii” - Abu Bakr had an ancestor named Sul, whose descendants were called the Suli. So, all in all, the name doesn’t help us much and could be one big coincidence.
While the name suli doesn’t give us much to work with, we are fortunate that the concept of mortals descended from jinn is not new. Perhaps most famously, some Islamic legends say the Queen of Sheba’s mother was a jinn. She certainly isn’t alone either; scholars such as Jalaluddin al-Suyuti and Ibn Taymiyyah write of intermarriage and procreation between humans and jinn.
Given that jann are the genies who spend the most time on the Material Plane, it shouldn’t be surprising that suli are among the most common geniekin. Consider which historical or contemporary figures in your games have verified or rumoured suli heritage. How do these elemental ties influence those people? Do they have political connections or responsibilities they must tend to on the elemental planes? Are they curious about their immortal heritage, or do they fear it?
Don’t forget that in the Age of Lost Omens, geniekin is just a label, so not all suli are decedents of genies. Sulis are also born when multiple elemental forces are mixed within a single mortal. In the Planescape days, tieflings often mixed several fiendish bloodlines together; this is the elemental equivalent. Planar sulis may not know their exact lineage, not because they’ve been abandoned, but because it is a convoluted mess of a family tree! Uniting several elements in a single being opens the opportunities for visual contrast, so while most sulis blend into their mortal heritage, don’t be afraid to get weird with dualborn suli!
Among Mwangi Pathfinders, h’Tuumba the Sootscale of the Mbe’ke is the highest planar authority. Born to a lineage of red kobolds and blessed by the cloud dragons in his clutch, the dualborn suli specializes in overlays between the Inner and Material Planes. Unfortunately, while he has an excellent rapport with planar dragons, he has alienated every single genie empire, all of whom have standing arrest or execution warrants should he be caught in their territory.
Demoactus the Seer was destined for greatness. His cyclops mentors prophesied he would be Iblydos’ next hero-god, acting as a conduit between the four elements. Unfortunately for Demoactus, he fulfills the prophecy just days after Aroden’s death, so the mythic trials meant to fuel his apotheosis merely transformed him into a suli. Humiliated and embittered, he fled to the Inner Planes, where he served as an envoy between the Elemental Lords for a century. Now he has returned to the mortal realm with an army of evil elementals in tow, bent on razing the island he was foretold to protect.
While merfolk are wary of species, enough of them in the Obari sea consider genies to be sacred spirits that jann merchants are welcome in their cities. Many suli merfolk live in Babras, Garden of the Sea, including their queen, Jullan. The queen is a former adventurer and knows many of the hidden passageways to the First World, Shadow, and Inner Planes, but she will want aid in her feud against Ragadahn in return.