I plan to roll a priest on the alliance side, while I know that there doesn’t have to be a direct link between gameplay mechanics and roleplay, I find I enjoy myself more when I can get immersed into any particular spec I’m playing.
So for that reason, I want to roll a priest that makes good sense lore-wise to play in any of the three specs. I currently have my choices narrowed between a Human Priest and a Dark Iron Priest. Although I am open to suggestions for other races of the Alliance
Worgen? Since they used to be humans, they probably know the Light, and then maybe after becoming Worgen, they “opened their eyes” to Shadowy powers?
Gnome? They seem to like experimenting, and being practical, maybe they’d see Light & Void/Shadow as just another tool to use?
Mechagnomes seem to me like they wouldn’t feel very strongly about neither Light nor Void/Shadow…
Dark Iron most likely
Dark Iron 
They have really cool aesthetics and I feel it might keep you more engaged than a human.
Dark Iron dwarves are open about using shadow magic. One of your companions in the Legion class hall quests is a dark iron shadow priest. Moira, as a world boss NPC, is using mind blast on you. And there used to be a prominent Twilight’s Hammer presence in Shadowforge city.
For humans, less sure. There’s the Twilight Father and cultists of course, but they are gone full shadow. Same with the Scarlet raven priests. Perhaps an angle like a “confessor” or similar could work for a discipline priest.