The Covenent will all have a unique Horde/Alliance model that will be used as the Allied Races of Shadowlands. We'll get new races every expansion but they'll be baked into Allied Races now.
Also Demon Hunters are really lame, the next class better be good if we ever get one again but when it comes to Races, we should expect new additions every major patch IMO so when the 5th Covenent on The Maw eventually gets revealed, players should get the option to switch/choose to the new 5th Covenent.