Introduced in the Dragon Empires Gazetteer/Primer and fleshed out in the Advanced Race Guide, nagaji are a slave race created by nagas to do their bidding. More stocky than serpentfolk, they are nonetheless clearly reptilian in origin. They are equally gifted at fighting and sorcery (and some of their druids can eventually metamorphose into true nagas). But as the nagas become more more and more insular in their obsessions, the nagaji only grow more numerous and more skilled.
There’s a lot to explore there. For one thing, what if different naga species favor different classes, abilities, or skin tones in their servants? For instance, dark nagas would likely discourage spellcasting—that would be too much like competition—but favor fighting brutes and rogues to help in their schemes. Lunar nagas might favor oracles, druids, and diviners who could help them understand the stars. And so on for guardian, royal, spirit, and water nagas.
Then there are always the adventure possibilities presented when a wealthy false theocracy rises on the backs of a poor religious underclass. If you want your naga masterminds to have armies of redshirts to hamper your PCs, you’ve got them. If you want your PCs to play a crucial role in the nagaji uprising against their false gods, you can do that too. Or maybe the nagaji have already risen up, or have simply outgrown their mud daub huts. That doesn't mean they want outsiders poking around in their temples, ruins, and sacred spaces, no matter how much or little they still revere their naga creators.
PS: If you’re looking for role-playing models, G’Kar from Babylon 5 leaps immediately to mind as a charismatic warrior-poet from a former slave race. He’s even stocky and reptilian in the bargain!
The Mouth of the Serpent Queen is the highest authority in the land of Naginka. She is also a spirit naga of exceptional foulness and evil. Adventurers are tasked not with publicly overthrowing her, but rather quietly replacing her. This means dispatching her out of sight, as well as smuggling the lunar naga priestess meant to replace her into Naginka—no easy task when alert nagaji guards watch every road and canal.
Searching for the source of the Yemu River, adventurers hire nagaji as litter bearers and rowers. All is going well—the nagaji even saving weaker party members a time or two—until a greedy water naga casts his covetous eye on the adventurers’ belongings. When the water naga slithers up to the barge and demands tribute, the nagaji instantly turn on the adventurers, explaining that no contract can countermand the will of a god.
A nagaji sorceress knows spells unheard of in more civilized lands. She is also being hunted by nagaji outlaws, the Cult of the Ghost Elephant. Saving her from this band of spirit-worshipping bloodragers and summoners will earn her trust and instruction.
—Pathfinder Bestiary 4 199
I’m not exactly shy about my love for 3.5 Forgotten Realms’ Serpent Kingdoms and nagaji fit right in with that book. It’s easy to imagine nagas trying to emulate the race creation of their sarrukh forebears. How the sarrukh, yuan-ti, and other scaled races respond to such effrontery from the upstart (and often half-mad) nagas could definitely be fuel for many adventurers as well.