Would you allow cyborgs in it? For example, a girl with mechanical limbs?
Hoo boy... I really ought to look at this rule book and see what other outlandish things are in there. Well, we've got mechanical prostheses in modern life, so I think it's conceivable to have Adalstier powered limbs and the like. Tentatively, I'd say it's possible, especially if you feel strongly on the matter. I'll need to read the rule book and figure out how things will work in terms of the game before I have a firm grasp on what's going to cause me a headache and what's going to cause me a massive headache.
My biggest concern is this; people will say "Why are cyborgs OK and Catgirls not?" I don't really have a great answer for why not Catgirls besides "adding more parts to a story is great until it reaches a certain critical mass, at which point additions don't bring any further vibrancy or development into the world and only serve to muddle the universe." So my biggest fear is losing coherence of this setting, distracting players and readers with too many moving parts and too many props. Cyborgs are easier to deal with; could just be prostheses for traumatic accidents and the like. Catgirls, other animal-human hybrids, adds a lot of stuff on my end since I'll have to start considering racial diversity (just how anthropomorphic is this campaign?) and all that jazz. Then the players and audience will undoubtedly want to know more about the cat people and what they're like, how they're different, what holidays they celebrate, their history, what were they up to when the First and Second were around, etc. And if those questions aren't answered, it begs the question of why have catgirls in the first place? Finally, zombie maids, vampire maids, devils/angels... all of those kind of conflict with the setting. This isn't a supernatural universe. It's Magitek. And I'd like to avoid mashing too many things together for all the reasons above.
So that's a very long way of trying to explain why I'm hesitant about introducing too many things. Of course, I want this to be fun for players. And that means letting them have characters they feel connected to and care about. It means letting your avatar be a representation of yourself and your personal creativity and likes/dislikes. But I also want this story to be internally consistent and without too many distractions from the main point. There's a balance to be struck here for sure and I don't want to start this whole endeavor with a heavy hand just banning stuff left and right like some scum GM.
For now, if you can justify whatever character trait you want within the framework of Antiope as I've described, feel free to do so. Cyborg limbs powered by crystals? That could fly. A zombie maid when I already stipulated that dead maids are dead maids? That's probably going to be a problem. Use your good judgement; I'll leave everything on the table and let players handle themselves with thought and maturity to fashion characters they think would fit well for the campaign.
I Trust You (All)!