Jinyu? It may happen but Metzen said he would have loved naga as playable race and Imo they could think up some lore to fit naga as a neutral playable race, with maybe two clans, one joining the alliance the other joining the horde.
But against the burning legion... TA-DAH! Furbolgs were well known in the past to have fought the legion at least twice. They are perfect, trying to repel the corruption of their race and giving powerful fighters for Varian's legions. Perfect.