Just playing around with ideas. Plenty of people weren't happy with some of the allied races chosen and some races have multiple offshoots or relatives part of the factions.
Here's my list:
Human - Tirasians | Tideskorn Vrykul
Dwarf - Dark Irons | Wildhammers
Gnome - Sand Gnomes | Mechanized (Tinkerforged?) Gnomes
Night Elf - Void Elves (Ren'dorei)
Draenei - Lightforged | Broken
Worgen - Kaldorei Worgen | Sethrak
Orc - Mag'hars | Ogres
Troll - Zandalari | Revantusk
Tauren - Highmountain Tauren | Taunka
Blood Elf - Nightborne (Shal'dorei)
Forsaken - Less Decayed Forsaken | Skeletons
Goblin - Drogbar | Vulpera
Pandaren - Furbolgs (A) | Mogu (H)
Most Tirasians are just normal humans and use the playable human model, but I think giving Alliance players the stouter human model would be a good thing. Vrykul are also a heavily requested race, they would obviously have to undergo minor remodeling and would have to be scaled down to tauren-size in-game.
Sand gnomes would be a fun variation for Blizzard to play around with in terms of culture and appearance. Mechanized gnomes is of course something that @shoc proposed. Gnomes with mechanical parts as a replacement for limbs, eyes, etc.
I was conflicted between Drogbar, hobgoblins and gilgoblins, but ultimately I chose drogbar because of their connection to Highmountain. They would obviously need remodeling and I'm not quite sure which skeleton they use.
Furbolgs, which can be remodeled and translated over to the pandaren skeleton imo, are already associated with the Alliance (the Stillpine of Azuremyst, the Barkskin which swore fealty to Tyrande and the Blackmaw which unsuccessfully tried to join the Alliance but we're sabotaged by goblins). I chose mogu because of the Zandalari possibly being able to convince them to at least join the Horde in an alliance of convenience. I also chose them over tuskarr and tortollans because having too many animalistic races would be on par with having more elf races. While mogu are loosely (partially) based on Chinese guardian lions, it's less apparent.
Tell me what you think.