I think when v42 kicked off, there was talk that any custom secret could be written down and taught to any fortress citizen using libraries. The wiki even claims that you can obtain a necromancy slab in Adventure Mode, leave it in a fortress and then reclaim that fortress and your citizens will all turn into necromancers. Seems to me there is much potential in such a system, but custom secrets are still a secret to me and the writing/knowledge system seems very unreliable.
OUTSIDER_CONTROLLABLE is a creature token instead of the civ token INDIV_CONTROLLABLE. So you could say, play as a dragon in adventure mode without having a dragon civ. There is also LOCAL_POPS_CONTROLLABLE and LOCAL_POPS_PRODUCE_HEROES. The exact usage is not clear to me, however, it is part of the system that allows wild animal folk to join civilizations. I don't think animal folk ever show up as migrants, but they can show up as visitors or in sieges against enemies and they're pretty common in adventure mode. I remember getting besieged by a group of zombie owl men that flew over my barricade in one of the earlier releases before zombies got nerfed (praise the toad for that btw) and thought "so that's a thing now, huh?"
Anyways, in general, it would allow you to make playable races that join existing civs instead of forming their own civs. I feel like we once discussed making a friendly megabeast like a 70s godzilla or gandalf type thing. I think you could use some combination of the above tokens to make that happen. Not sure if it would work on a legit [MEGABEAST], but it would probably work on a yeti-like creature. Might also work for talking animal friends or spike-the-dragon type characters, but those might be better off as pets. There might be some way to statistically guarantee that a certain civ develops a minority population of a specific race.