I doubt Blizzard is going to reconsider any of the initial six Allied Races they have shown off as major features during BlizzCon. Plus, with the Alliance getting a Blood Elf model (granted, not High Elves, but a playable Alliance BElf model nonetheless), the Horde needed something shiney to offset the outflow of Void Elf re-rollers from the faction. The Nightborne and the Zandalari might even the scale versus Void Elves and the Dark Iron. Highmountain Tauren and Lightforged Draenei are pretty much expanded customization options and in my opinion would not affection faction balance in any meaningful way.
Quick stream of consciousness before I jump to the “allied races I wanna see”: out of the initial Allied races only the Dark Irons and the Zandalari make sense. The dwarves are already a part of Ironforge, even if grudgingly so, and the Zandalari needed a quick “But these weren’t the same Zandalari from the MoP!” retcon to work, but with Vol’jin’s touch they do make sense. The other four currently cause more questions than provide answers, but there is nothing I can add to the thread that hasn’t been beaten to death yet. So… here we go!
For the ones below what I set as sorta guidelines for myself is a) the same skeleton / model (in most cases) b) the lore development for the race and for the faction they belong to c) mirror development for the other faction
• Orcs: Mag’har tribe from Outland
• Draenei: Kurenai Broken from Outland
In terms of lore, both sub-groups already belong to their respective factions as long-time allies and would bring news of what is going on in the Outland as of now. There is, however, a rather big minus in there. WoW doesn’t have a Female Broken model, I believe
This gives the Alliance a new model, while the Horde gets technically more Orc customization options and recolors, but I believe it can be offset by switching the tables in a different set of Allied races.
• Dwarves: Wildhammer clan
• Trolls: Revantusk tribe
The Wildhammer clan is the old friend of the Alliance, while the Revantusk have declined the invitation to join the Horde, but continued working with it nevertheless. With Wildhammer pushed out from the Twilight Highlands in Legion (Stormcaller Mylra, a Shaman follower, says that in the Class Order Hall), both groups would be present together in the Hinterlands. In the aftermath of whatever happens in BfA, conflict seems un-avoidable, which might make Revantusk reconsider the partnership with the Horde. Provides lore on possible fighting in Hinterlands. Dwarves get more customization (tattoos!), while Revantusk look the same as currently available Darkspear and Shatterspear trolls. However, perhaps, it can be “fixed” by giving them new hairstyles, tusks, skin colors or tattoos of their own.
There can be some difference in the offered power since Wildhammer was opposed by Dragonmaw orc clan (almost destroyed through SoO), and Revantusk are but a handful of Amani trolls who are not even fighters.
• Worgen: Bloodfang Pack
• Forsaken: Undead Elves / Newly risen (Chinese Forsaken model)
The Bloodfang pack lead by Ivar Bloodfang was attacking the Forsaken in Silverpine Forest (and Shadowfang Keep) and later in Hillsbrad only to never be heard from again. With the Siege of Undercity and the recurring fights over the territory in that area, they might be worth remembering and re-integrating into the Alliance (to provide the Worgen more customization, of course!

). For all we know, they could have been having sneak attacks on the Forsaken of Silverpine all this time. Numbers-wise, they are probably close to the Revantusk.
Undead Elves speak for themselves, and while I would be all for this option, having third Elf model on the Horde might be weird. The other Forsaken option is un-inspired, but besides raising new people, I don’t see a way for the Forsaken to gain some more numbers and get customization options in progress

Both groups can participate in the fight for Undercity (post-destruction, after the Alliance pulls back and the warfront stuff starts).
Those ones seem like the most likely cases for me. The ones below I honestly can’t see happening.
• Dryads & Grove Keepers (as male choice) for the Alliance
• Centaurs for the Horde
Dryads & Grove Keepers (alongside with Ancients, Hippogryphs and more) are already a part of Night Elven forces who can be pushed out of the neutrality by the burning of the World Tree. With the conquest of Kalimdor, Desolace’s Centaur tribes can be recruited for the Horde… or be killed off.
Cannot see this happening due to the whole mount / flying mount / below waist gear debacle (alongside with the Naga), but one can dream, right?
I cannot honestly think of anything for Goblins or Gnomes. There are Mecha Gnomes, Leper Gnomes and Sand Gnomes, but I can’t see any reason for them to join the Alliance. However, it might be my lack of vision over anything. Same with Goblins and Gibgoblins.
* wheezes *