O-powers are back, which is cool. Something tells me the 2 Z Moves per battle would be limited to in-game only. Exp. and hatching O-powers were the ones I used most in Gen VI and those would ease grinding and hatching eggs in US/UM.
Anyway, the thought of offensive Stealth Rock Dusk Mane Necrozma is starting to sound scarier the more I imagine it, as it has a nuke in Searing Sunraze Smash and Photon Geyser as a stronger Psychic STAB than Zen Headbutt:
-Stealth Rock
-Photon Geyser
-Sunsteel Strike
-Earthquake
Nature: Adamant/Jolly
EVs: 252 Atk/4 SDef/252 Spe
Item: Solganium Z
Calm Mind Lunala would appreciate its personal Z Crystal over the Ghostium Z in order to still bypass defensive abilities unlike Never-Ending Nightmare:
-Calm Mind
-Psyshock/Hypnosis
-Moongeist Beam
-Focus Blast
Nature: Timid/Modest
EVs: 4 HP/252 SAtk/252 Spe
Item: Lunalium Z/Psychium Z (latter for Z-Hypnosis for +1 Speed)
Dawn Wings Necrozma can also pull off the above, minus Z-Hypnosis, as well as Photon Geyser as a possible Psychic STAB. Lack of Shadow Shield would hurt, though.