I agree that in your 4v4v4 version, night elves, blood elves, tauren, and worgen would make a certain amount of sense but it'd also be a bit boring. They'd work together too well. There would be no drama.

I think the worgen should stick to the Alliance and the blood elves stay over at the Horde. The worgen are formerly humans. The blood elves have too much invested in their relationships with the Horde and I don't think the night elves would welcome them too much, despite their redemption.

Instead, have the gnomes join the faction (I mean.. WHAT do they really care about the Alliance, anyway? They've always kind of looked out for themselves more than anything. And the third faction needs a shorty race - the Alliance would still have the dwarves, and the Horde goblins). On the other side, have the Darkspear trolls come over. The trolls under Vol'jin would obviously want out of Garrosh's Horde. They'd also want to learn more about druids alongside the races most known for it. And being the ancestors of the Kaldorei, no doubt the night elves will kind of feel a kinship with them.

The only problem I see is this would mean the Horde would be left without a single race that could be druids. Orcs, blood elves, goblins, and forsaken.