It might be excessively nerdy to answer the question, "why nagas?" ...Seriously though, what if humans, having limited resources to terraform a planet's ecosystem and climate, took the shorter route of altering themselves instead? I imagine nagas living happily on a desert world of extreme temperatures, able to lay dormant for extended periods of time, living off of the water and fat stored in their huge serpentine bodies and thriving during seasons with flash floods... scaly skin would be thick and resistant to sun, sand, bugs and scratchy plants. The main argument against them is how slow and cumbersome they'd be, but ambush predators make a pretty smooth living if they can wait long enough. I imagine hunters leaving the communal den for days at a time, able to sit still and go long periods without eating, coated in mud to blend into the terrain, as they wait for prey to come close enough. Real snakes do this on Earth. ...I need to spend more time on making my work look finished.