Instead of having the allied race be locked behind a rep and a achivement, they should just be locked behind the achivement. But they could make hitting exalted give you new classes and new customization options. These would have a small quest chain when you are exalted, and then you have them. And there is no grind required to play a race other than easy to earn achivements. The special things for each allied race would be…
Nightborne: Felborne customization and dh
High Mountain Tauren: Feltotem customization and dh
Void Elf: High Elf and dh
Lightforged: Eredar customization and dh
Mag’har: dk and new tatoos
Dark Iron: Mole machine customization and dk
Zandalari: Dire troll customization and dk
Kul’tiran: Thin Human customization and dk
Future Allied Races
Vulpera: Hopefully they get a caraven racial that is similar to Mole machine, then exalted would be dk and caravan customization
Sethrek: Spikes like sethralias has and dk
San’Layn: dk and orbs like kaelthas had
Ankoan(8.2 fish worgen): Jinyu customization and dk