IMHO I think the main problem with half-orcs, fluff-wise, is the assumption that the 'half' part means 'human'. Ditch that and you're fine, and it opens up a world of possibilities. And you can keep it that most people *think* that they're half human, half orc 'cause that's how they look, but really they're their own race.
In my game I've done that with half-elves. They're called 'half-elves' colloquially, but really they're half (or just part) fey. They could have had a nixie, or saytr, or even harpy ancestor. Means half-elves are much more fun, can all look interesting/unique, and you get out of that whole why are there no half-eladrin or drow, why don't they get a bonus to dex weirdness. And explains why they make such good feylocks.
I do the same with tieflings. Get the player to pick a devil and get physical/emotional charastics from that instead of just the horns and tail shtick. More fun. But each to their own.