I don't think this time we're gonna have a new name for Lumiose City and hence new regional forms. Even because the game is based on Lumiose City, so it doesn't make much sense to make new regional forms only available in one city. I don't Discard this option completely yet, i just don't think it's gonna happen. Instead we'll have mega evolutions.
Hisuian Pokemon, i think that they might be there, especially if the game is set in the past like PLA. Though, I am not sure how. I don't expect them to be available in the city, maybe some kind of secondary quest, like in SV DLC.
I am not sure about Pokémon HOME. It really depends on how the combat system is going to be in Legends Z-A. For example, Hisuian Pokemon might break the game? If so, i hope GF is smart enough not to allow them. Also, generally Legends is for me the kind of series that makes new Pokemon/forms to transfer them to the main game, that in this case would be the gen 10 game.