It would make sense in a way since HE's have been Alliance-affiliated throughout the entire game, plus given the whole 5.1 pre-Sunreaver incident where Varian was actually in negotiations to bring BE's into the Alliance fold before things went crazy in Dalaran. (A way to show some sort of progress without any actual progress being made!
)
But yes, since Horde would have to be given something of equal value, I honestly don't know what would fit them from the Alliance races.
Humans? They already have Forsakened Humans. Some subset of Vrykul? Syndicates? Dunno.
Dwarves? Dark Iron is the only Dwarven faction that isn't staunchly Alliance-affiliated, but with Moira sitting in IF and all, not sure why they would swing to Horde.
Gnomes? Leper Gnomes are the only 'gnome splinter faction', and Horde already has Goblins to fulfil the 'short race' quota.
Night Elves? Highborne was the only real splinter faction, and they were re-integrated into Alliance in Cata.
Draenei? It's temping to say "Give Horde Eredar!" but Draenei already
are the splinter faction of Eredar, not the reverse, so this doesn't really make sense. Although, given what the Orcs did to the Draenei on Draenosh during their ZOMG DEMON BLOOD RAGE RAAAARGHRTHGHGH! days of WC1/WC2, you could say they could either go with Draenei/Orc hybrids (ala Garona's retconned lineage) or just Broken. It would be a stretch either way, but at least it would use the other BC race for 'balance' sake.
Worgen? Don't really see it. Forsaken siege of Gilneas pretty much drove them directly into the Alliance, and the Worgen were supposedly immune to the plague or something, so you can't even say "Forsakened Worgen" or something silly. (Zombie Werewolves. lol)
Now, if you didn't care about trying to find an existing Alliance subfaction that would make sense in the Horde, the obvious Horde race would be Ogres, since they were the heavy hitting Horde units in WC2 (WC1? Dunno, only played WC2 and WC3)