Since Z-A appears to be set in the Present (around the same time as SV which would be many years after XY) Lumiose city here are some characters I could see showing up.

Do not that at the end of the trailer the Team Aqua and Magma logos can be spotted implying they are back in some form.

Hop is the 3rd most popular SwSh character in Japan and would tie back to the first legends game.

Alian is Cannon to the games in some form

And green just wants to catch mewtwo and silver is tied to her heavily in the Adventures manga we're she originates from.