For me, it's as follows.
Name: High Elves
Reason: High Elves are an established Alliance race and heavily involved in the lore. The fact that they are a shattered race with no home and no clear leader only makes them more compelling. While the former lieutenant to the old ranger general is regent to the Blood Elves, I doubt most High Elves would bow to his marshal law, even less so because he's reduced himself to subsisting on demonic energy.
Name: Half Elves
Reason: Half Elves are normally abandoned by their Elven parents, it seems to me that they would be already part of the Alliance.
Reason: Some Mok'Nathal have already joined the Horde, namely Rexxar, they should be playable.
Reason: The Ogres have long been a part of the Horde, while most other races consider them to stupid, some Ogres have shown a surprising degree of intelligence. It's not to farfetched to assume some of these might become heroes of the Horde.
Name: Forest Trolls
Reason: At one point, the Forest Trolls were a part of the Horde under Zol'jin. Now they are in disarray. I think it would be cool if they rejoined the Horde under the Darkspear banner.
Faction: Horde, Neutral
Reason: I think the Tuskar would best integrate with the Horde, mostly because of their simple shamanistic beliefs, although arguments could be made for their neutrality. There is the unfortunate problem of them dying in warm weather. But this can be retconned as needed.
Reason: I'm torn with the Wolvar They have the ability to craft tools and weapons, making them competent mercenaries, but their lack of sophistication and warlike tendencies hold them back from allying one way or the other.
Reason: I'm not talking about Worgen. Werewolves existed in Warcraft lore long before Arugal summoned them to Azeroth, as seen in WOTLK in the Howling Fjord. It's not entirely unreasonable to assume a group would join the Horde to spite the Worgen, indeed, the latter's appearance throughout Azeroth may provoke the legendary creatures out of hiding.
Reason: While the powerful and dominant Mogu may seem more suited to the Horde than the Alliance, the Mogu are intelligent, developed a sophisticated society and respect strength, which can be found in the Horde and Alliance in equal measures.
Reason: The bestial Gnolls have plagued Human lands since time immemorial, with the Humans consolidating their holdings in Elwynn, it's only a matter of time till the Gnolls are driven to the brink of extinction. Will they look to the Horde for support? Some are intelligent and capable on an individual level, it seems plausible.
Reason: The Centaur hold no love for the Horde, despite the obvious animation problems, they would otherwise make good members of the Alliance
Reason: Not likely, but it would be cool.