The set will introduce a brand new mechanic called Prism Star cards. Six of these Prism Star cards will be made for the set. No further information is known about them.
“Prism” is often a type of holo rare in other card games, so it wouldn’t surprise me if this new mechanic is also a new type of rarity. We’ve had prism-esque holos before (like Starmie BREAK) and “star” cards before (like CARD NOT FOUND). “Prism” almost certainly refers to Necrozma, the Prism Pokemon, so its two forms will likely get Prism Star cards.
There will be ten Pokemon-GX in the set, five of which are Ultra Beasts. All of the Sun and Moon Ultra Beasts have been released in the English and Japanese TCG, so unless two of them get new cards, this means there could be two more Ultra Beasts in Ultra Sun and Ultra Moon aside from UB Burst, UB Adhesive, and UB Assembly (unless the two Necrozma forms somehow count as Ultra Beasts).
Over 150 cards will be featured in the set, as it will be a combination of the cards from Japan’s December SM5 set in addition to the exclusive cards from this week’s SM4+ set. The set will have 18 Trainer cards and three special Energy cards. Despite being marked as “Ultra Beasts” on their cards, no Trainer cards or anything else in the game targets Ultra Beasts yet, so we’ll probably get some cards that support them.
Like Japan’s upcoming SM5 set, the set’s mascots are Dawn Wings Necrozma and Dusk Mane Necrozma. Each will get their own Elite Trainer Box.