My best guess is that everything pre-BREAK will rotate, so that leaves us w/ BKT onward for the beginning of next season.
Staple Supporters (i.e. supporters in just about every deck) won't change too much this rotation: Sycamore and N will still be the big ones. Guzma will instantly replace Lysandre. And Acerola/Olympia should be one-ofs, much like AZ was in the past. So that's the core. Along with Random Receiver, which will initially slide into VS Seekers place. Lele has already replaced Shaymin for all intents and purposes, so that's checked off the list. Beyond that, it's all speculation.
Personally, if I'm putting my prophet hat on, I think Octillery and Oranguru will find themselves in almost every deck, which makes cards like Mallow and Puzzle of Time interesting because they reorder cards and boost consistency. Puzzles also give you recursion from the ditch, which decks will be wanting once we no longer have instant access to all of our Supporters by simply Ultra Balling them into the discard pile.
Misty's Determination is very intriguing. As is Skyla. Looking at deck lists from a couple years ago, cards like Skyla were played in multiples, so we could be moving toward that. As GX attacks become more prevalent, Hala's usefulness will increase. Lillie's T1 draw abilities are interesting, but since Garb has made emptying your hand T1 not as valid of a plan, I don't expect it to see much play.
I still expect tech mons, like Hawlucha or Oricorio just to name a couple off the top of my head, to be played more and more, which would make Ninja Boy more common.
So far, so spookiest card that's being rotated (unless we get the full art as a secret rare or a promo) is Hex Maniac, as it will allow ability-based decks to run rampant and be abused, much the way tool cards were abused up until the release of GRI.
That's all I can think of atm. I'll prb think of 20 things while I'm trying to sleep tonight and check back in lol.
