Sethrak of course.
I would likely say they should be neutral.
As far as the model goes, only some slight updating to the eyes. Not a lot since snakes do have fixed eye sockets and can only move their eyes so much.
Maybe add some extra hood patterns and snout types. (Ie: Default (what they already have) Hognose, Boa/Python, Patchnose Snake).
Different tail options: Rattle, no Rattle, Spider Tailed Viper
Otherwise their model is already really well polished.
Starting mount would be a Bazaar Krolusk (similar to the Caravan Hyena)
Starting Hunter pet also Krolusk.
Classes? Hunter, Warrior, Rogue, Shaman, Warlock, Mage, Priest and Monk. Perhaps Druid due to the nature of the Loa Sethraliss.
Active Racial: Venom Spit with cripple or nature dot.
Passive: Either a small buff to critical strike or haste.
What race would I pair them with? Probably the Mogu. Cause thunder and lightning theme!