This will be a really really good card: or at the very least something I will use!
I'm already playing the Breakthrough Magnezone in Raichu (it does the same thing but for electric types).
Maybe this will see play with that super hyped Solgaleo deck (which will likely also include the Dawn Necrozma)
I think this is good in Silvally and Metagross, or disruption Ninetales.
I might also throw this into Gardevoir. Heres the combo: Guzma the Pokemon you want them to keep and put up Float Stone Octillery, play Parallel City, Play Cyrus, Retreat and Plea GX. Evil laugh!
I'm excited for Prism Star cards now that they are not limited to only Pokemon. I really like the idea of limited, but very powerful, cards. I hope they won't be too rare or valuable- but I am anticipating 1 or 2 of them a box, maybe.
I like your list a lot!
I would say, in my opinion, to take out:
-1 Bridgette
-1 Field Blower
-1 Choice Band
Put in:
-1 Registeel
-1 Acerola
-1 Cobalion (It's really good!)
Not 100% sure but overall good. I don't know how important Magearna EX is to this deck, it is a bit overrated.
I also...
I think it's a good list, but decks always have room for improvement. I would say take out 1 Kartana, a Skyla, both Fighting Fury Belts, and add another Registeel, another N, another Sycamore, and maybe a Counter Catcher.