I guess I can understand the issue with holy void elves, but with the exception of holy specced void elves, it isn't really stretching lore. Discipline works because it mixes shadow and holy into healing--I could definitely see a void elf priest going in that direction. But would void prevent them from using holy powers? Would it weaken them? That lore is vague at best, but I'd say that it would not. Void and holy seem fairly interchangable, so they don't really prevent eachother. That said, why would a void elf be a holy priest? That makes little sense, unless they just regret everything and were trying to atone for it. So your point largely stands, but I wanted to touch the topic since I've heard it before and it interested me.
Either way, I feel like with lore, the damage can be manageable. You can destroy a story or you could risk a few plot holes that are relatively safely ignored. With goblin shaman, I don't know. I mean the lore is there, it just feels odd all things considered. If we're talking about druid mechagnomes, don't worry... I know they aren't going to happen. It's just a rare case where I wouldn't mind seeing lore stretched to have them playable, even though it makes little sense. The lore breaker there wouldn't be their existence, but of druid order hall forms.