I made a similar post about this last year. It’s still relevant if you want to have a read.
Honestly, opening the base classes (those that start at level 1) for everybody would be best. If not everybody, at least the original races.
Pandaren are particularly hurting for class options (and only have two tanks). They would benefit greatly from paladins, druids, and warlocks, and it’s a great opportunity to widen the scope of the lore and throw in some humor (such as the Terrible Turnip infernal).
As far as Allied races go, they need some warlocks. The Lightforged in particular could get an inquisitor class with the force-converted former agents of the Legion (lightforged demons) as their minions. It would be mechanically identical to warlock, but spell and icon flavored to the Lightforged’s crusading aesthetic.