when Writing Bull described your world as a world where old superstitions have become true, I had an idea. I suggest that Goblins steal human babies and raise them as their own. So very rarely a group of goblins might have a human in their ranks, with visuals that make him fit into their ranks and AI behavior and perks exactly like a goblin skirmisher.
Also you could make tavern rumours about those ‘Wechselbalge’ and a mission type like ‘Goblins have snatched the Mayor’s infant kid’.
Oh my god how about some “Der Erlkönig”? What if, the Goblins were transients stealing money from all the villages, without a national homeland of their own? Pretty good. Bigger noses.
But more importantly, Elf children stealers!