I'm hoping W4 changes the factions. I can't imagine the tauren sticking it out with the orcs forever. I believe it was only the bloodhoof tribe they saved, not the entire species. Theres a couple of different ways I can picture thing going. One way is three different factions instead of two. I know one of them would be night elves, draenei, worgen, and tauren. Possibly even the blood elves depending on how the return of the highborn went, since W4 will be somewhere in the future. I don't know who else would be allied other then humans, dwarves, and gnomes, but having three factions could be fun instead of just the normal two since wow established closer ties and opened the world up a lot.
Another way I can see this going is everyone gets sick of the forsaken trying to wipe out all life on the planet so they band together to destroy them. After that the orcs and humans, being the dumbasses that they are, will then continue their war against each other. This is when I see the rest of the races on azeroth basically collectively saying "You know what? Fuck you guys" and then banding together to wipe out the orcs and humans. So then the orcs and humans will have to partner up in a fight for survival.
I'm hoping its the second scenario I imagined because quite frankly, I am sick and tired of orcs and humans. Its a damn fantasy game, I don't want humans to be 90% of the content I play as alliance. The horde side isn't as orc dominated as alliance is human dominated, but its still annoying that a big portion of it revolves around the orcs.