You know one past gen pokemon I could see potentially getting a Signature Z-move in Ultra Sun and Ultra Moon is Celebi, I say this because if you look back in Pokémon Sun and Moon Mew wound up getting an signature Z-move and the virtual console releases of Red, Blue, Green, and Yellow wound up being compatible with Sun and Moon where you could transfer Pokémon from those games to Sun and Moon through the use of Poke Bank, now we have Gold and Silver coming out for the Virtual console and of course these will be made compatible with Ultra Sun and Ultra Moon as well as Sun and Moon, If I'm remembering right I think the update for Pokebank to allow people to bring in pokemon from Gold and Silver for the virtual consoles is coming out in December, although I could be wrong about that release date though.
So I wouldn't be that surprised if through Pokebank they have an even where you can get the Signature Z-crystal of Celebi(that is if there is one), which interestingly enough there is an event where if you get either Gold or Silver you get a free Celebi for Sun and Moon and Ultra Sun and Ultra Moon.