Yeah, demon could be one to be added. Demi-god I actually considered as a class (it's actually in the database for the game) but it would have to be a New Game + unlock so it doesn't ruin the balance. I'm not sure what the new races will be though, I'll be thinking about and see if I come across something.
I could add an adventurer rank based on guild quests you've completed. I'm not sure off the top of my head what the benefit would be. I don't want to lock quests behind rank, giving an item/gold doesn't really seem fulfilling. Maybe it could give access to a home at a certain rank or give store discounts (though I think I would need a plugin for this). I'll think about what the benefits could be.