Hi guys i can help you with the lore.
First of all Theramore was founded by Jaina after Lordaeron fell and the centaurs aren't united at all and as an important note in WoW as both the Alliance and the Horde you get to ally with one of the Centaur factions to keep them fighting each other and they don't have slaves.
As emergent factions you could have the Forsaken and the Blood Elves.
Guys the Steamwheedle Cartel should not be a playble faction because the goblins want money not to make wars.