Figured it is time for me to finally join this thread. I am a HUGE advocate of adding the races that we know are story-wise already in the factions. Old allies that should have been added years ago.
And just to preface this: I do not agree with adding any as customization unless they upgrade how they do it.
Horde
Taunka - Joined the Horde back in Wrath of the Lich King, led by Roanauk Icemist. This is perhaps my most wished for playable race! VERY possible in the Last Titan!
Ogre - A staple race of the Horde since the start, it is criminal they have not been added yet. We have various clans in the Horde, including the Stonemaul, Dunemaul, and Ogres from AU clans that came with the mag’har. ATM no currently announced expansions scream “Ogres”, but hopefully soon!
Forest Troll - While most Forest Trolls no longer support the Horde, the Revantusk Tribe have continued to be staunch allies of the Horde. They reconfirmed that in BfA, and could be added in Midnight when we revisit their homeland!
Mok’nathal - Half-ogres from Outland, they recently expressed interest in perhaps reconnecting with their kin in the Horde. Currently no announced expansions would seem to obviously include them, but hopefully we see them gradually appear going forward!
Hozen - The Forest Hozen have been loyal Horde warriors since the time of Mists of Pandaria, most recently appearing as soldiers in BfA (Alive and Skeletal). Main obstacle would be how their anatomy is structured, though with a revamp we could see it become more playable in style.
Gilgoblins - Former slaves aided by the Horde during BfA, we have seen them journey to Orgrimmar and keep relations. Perhaps they will fully join the ranks of the Horde one day!
(Honorable mentions that are unfortunately added as “customization” already, Darkfallen & Farraki Trolls)
Alliance
Broken/Krokul - A marginalized part of Draenei society, we have seen recent developments to bring them formally into the mainstream Draenei society once more. This would be a perfect opportunity to make them playable! Perhaps as a counterpart to a Horde AR in Midnight.
Frostborne Dwarves - Allies of the Alliance since Wrath, would be a perfect Allied Race in the Last Titan as a counterpart to the Horde Taunka. They most recently appeared in Exploring Azeroth: Northrend, where Muradin formally invited the frostborne to Ironforge.
Jinyu/Ankoan - Allies of the Alliance since Mists of Pandaria and BfA respectfully, they would be a great counterpart to Hozen or Gilgoblin one day!
Furbolg - Ancient allies of the Kaldorei, and most recently the Stillpine Tribe has had close relations with the Alliance overall. They most recently appeared in the Draenei heritage armor where they reconfirm their friendship. Seeing them become formally playable would be great one day!
(Honorable mentions that have been added as “customization”, High Elves, Wildhammer Dwarves, Man’ari Eredar, Darkfallen).