So just some quick thoughts from me:
Typing & Ability: No surprise it keeps its typing, it's essentially Solgaleo and Lunala now. Prism Armor is a nice defensive Ability, being a Steel-type Solgaleo would certainly appreciate it (especially to deal with its Psychic-type weaknesses) though Lunala would probably had prefer to kept Shadow Shield though this might help keep it from getting OHKO from its quad weaknesses.
Photon Geyser: As everyone said, nice they finally made a move that goes off a Pokemon's highest offense stat. With all the Legendary pairs (or trios) who share a type and have one member Physical focused and the other Special focused it's kind of surprising they just now decided to give them a shared Signature Move that does that.
Anyway, guessing it'll either be instantly learned by Necrozma when it fuses or be a level-up move (when we battle Necrozma it'll probably already come with it).
Solgaleo & Lunala Signature Z-Moves: Hmm, wonder what their Z-Crystals would be called. Solgalium Z and Lunalium Z? Yet Necrozma can use it, and the Z-Crystals use the Sun & Moon symbols instead of directly relating to Solgaleo & Lunala. So maybe it'll be something a bit more generic like Sunium Z (or maybe Sunnium Z since Solgaleo is the Sunne Pokemon) and Moonium Z?
Anyway they give off a nice spectacle. Don't know why they suck the opponent into Ultra Space but there's no kill like overkill I suppose. Though I did have a laugh when Necrozma Dusk Mane (and I'm also going to guess Solgaleo?) rolled up into a ball to do its attack; does that mean they can now learn Rollout? Also I like how they show Solgaleo jumping onto a floating crystal after the attack so it looks upon the destruction, wished for Lunala's it cut to a view of it watching the light show. Though once again, I think Lunala's get the better name. Also, like the moves they'd need to activate them, they have Mold Breaker as an additional effect which is always nice (though does mean, if put against each other, Dawn Wing Necrozma has the advantage over Dusk Mane (and normal) since its Signature Move is super effective against it and can't block the damage with Prism Armor).
Rotom Dex & Roto Loto: I like the idea of getting to actually befriend Rotom Dex, considering he'll be staring at us through the entire game once we get it. Be less creepy. Curious how it'll "change" depending on your decisions, and are these changes permanent or can doing actions in the post game change them (like can we have conversations with Rotom Dex or are these just sudden choices). But hey, any character development is appreciated after all it did in the original Sun & Moon was sit back and made snarky comments while telling you the same information you were just told. I also hope this means in the post game it'll have a lot more things to say, like every time you boot up the game it's either telling me to fill up the dex (which I already did) or wondering how Lillie or Hau. Like, have it comment you about your Battle Tree or Battle Royal record. Maybe have it give us objectives to complete to help it do research it's doing like catch/battle a certain amount of Pokemon, use a certain item, etc..
So O-Powers are back in the form of Roto Loto "items". We get a screen of a few of them and they're pretty standard: Roto Hatch (hatch eggs faster), Roto Bargain (50% discount in Poke Marts), Roto Prize Money (raise prize money after trainer battle), Roto Exp. Point (Pokemon get more experience after battle), Roto Friendship (raise a Pokemon's Friendship faster), Roto Encounter (increase encounter rate), & Roto Stealth (decrease encounter rate). But then we see a new one: Roto HP Restore. Don't recall a O-Power relating to that so mystery what it does. Does it completely restore the HP of one or all your Pokemon? Does it increase the healing effect of potions? While walking does our Pokemon passively heal themselves?
Also, I noticed you had the choice of giving the item to a Pokemon. Assuming this is just not a default function of the menu and it does nothing, what would giving the Pokemon the item do? Is it meant to be sort of a gift thing, you give a Pokemon a Roto item and then trade with another player? Or, since they look to have not implemented the O-Power's ability to give it to another player (or at least what we're seeing, for all we know we can use them in Festival Plaza on a guest but that's still rather limited compared to the PSS), would giving it to a Pokemon means only that Pokemon will get a boosted/longer lasting version of the item so to take advantage of it we'd need to have that Pokemon in the front of our party (and some items like the Prize Money and Experience would require it to battle or only affect it). Also, since these are items I'm guessing they're one time use? Convenient they hide the top screen which would show you how many you would have, though this would explain why you get them via a slot roll. Wonder how you get a slot roll to happen, maybe Rotom Dex now too gets experience from battles but instead of leveling up it does a Roto Loto each "level up"?
And indeed, it just looks like Rotom Dex can let you use Z-Powers again. I was wondering if there was a limit, like you can only re-use the Z-Move you used, but it doesn't look like that's the case. It's a free Z-Move recharge in the middle of battle. (Enter your complaints about making an already easy game even easier here). I actually don't use Z-Moves that much anyway, I'm honestly more interested in the Roto Loto.