Here’s my thought:
Recently we have been introduce to a plethora of new creatures, ranging from the original goblins to the newer Kobolds. What if there was some kind of “hybrid” species? I suppose it might work something like this:
Later in the game, some friendly species may be introduced. Maybe even a possibility of “friendly goblins” or something like that. With these ally clans, the hearthlings and, say, goblins can procreate (This is a very general term, which may or may not be some weird spore-like dance to reproduce) After, the result can be a hybrid of the two species, maybe a “hearthgoblin” which has aspects of each species. For example, a “hearthgoblin” might have the general mental strength of a hearthling, but its goblin side makes it more ruthless and aggressive. Another possibilty is if one was able to take a goblins garbs and wear them, granting them some of the abilities a goblin has.
This is a rather complicated concept, so it might be better to put into action after the game is completed, so it can be an update rather than a beta/alpha addition.