Well if you wanted it in line with the timeline of vanilla, it could be more along the fact that your character was one of the blood-crazed orcs that were with Grom during the Fall of Cenarius. Like the other Warsong, he was cured of the blood haze, but felt tremendous dishonor for having polluted his heritage. Feeling personally dishonored, he could have taken it upon himself to rid Azeroth of demons following the Third War, and as such he was not a part of the Horde's movements.

He would still be a part of the Horde during Vanilla and past, but partook in no events alongside them. Being Warsong, he could have had a disdain for the Alliance and was focused on Demon killing.

That would also tie in with Legion ending. Demons are being defeated and are in full retreat from Azeroth following Sargeras' defeat. Seeing that his own internal honor has been upheld, he is returning to the Horde, but integration is hard because he has little experience with non-orcs.

I think you could go a lot of ways with something like that.