That'd never happen because Humans ARE the Alliance. Orcs VS Humans. Besides that, Alterac only joined the Horde because of Deathwing, and that was the old Fel Horde. Most of them went on to become the Syndicate, which favors Orc slavery and is hostile to the Horde.
It would be far more likely to be dwarves, since they already have a slightly evil, cantankerous subfaction that fits in with the Horde. It might even be why Blizzard picked Dwarves to be the first race to get a new model, despite Orcs and Humans being the main races, and looking just as old and clunky. Pair them with High Elves to reap the race change $$$, then again when the new human model is released and a lot of people change back.