tauren rogues (grimtotoems) already exist just not playable. they're 'one with the land' hooves don't matter.
Night elf paladins. no more pallys in the horde please.
Forsaken paladins, pls.
The one combo that I think would make the most sense would be Pandaren Druids. I'm surprised it wasn't in game already and I hope that it is the one race/class combo Blizz considers for the next expansion.
Gnome hunters, if the goblins can do it, why not the gnomes? And make goblin & gnome hunters be allowed to use their pets (depending on the pet they have) as a mount! I'd so badly race change (and faction change too ofc) my blood elf hunter to a gnome if they can become a hunter, it would be so funny to see
What is there to combo without screwing up the whole lore of WoW?
This, hands down, is the race/class combo I would most like to see added.