The core formula for allied races is you meet the race/faction, do quests and gain rep to get their respect and trust, and have them join your faction.
Why we won't get Kul'Tirans is because we already sail to Kul'Tiras in 8.0, do their quests, and have them officially join the Alliance. And we already know in 8.0 the only new addition of allied races are the Zandalari and Dark Iron.
If they become playable in a future patch, would that not mean we do a bunch of quests to have them doubley join the Alliance?
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Fingers crossed for Vykrul.