Is there a list of the cards that came out in the SuMo Japanese expansions, but were not released in English? Brooklet Hill golden card, Mew and Mewtwo tag team alt art, just as examples.
And follow up, do we think all of them are going to show up in Hidden Fates? I know Jessie and James full art is supposed to be in that set so I can only assume a bunch of others will be too but somehow I suspect not *all* of the unreleased cards.
@quuador any info here? I figure you may be the best person to ask
Hidden Fates will have the cards we missed in the last combined sets. Everything after that should be in our November set.
The mewmew alt art will almost certainly be a box promo similar to the reshizard one. Purposely delayed so people buy UM instead of promo boxes.
Not sure. I know of this page on Bulbapedia, containing Japanese cards not released in English (although it’s missing quite a lot of them).
As for your follow up: probably not; not all of them being in Hidden Fates I mean. Some of the cards are probably released as separated promos. I think the Shiny Vaporeon was already confirmed as a Tin promo on PokéBeach before we even heard of the Hidden Fates set a few weeks/months back.
I also collect Full Art Supporters in English and Japanese exclusive, and although almost all FA Supporters have been released at some point in English (except for the obvious Team Rocket Brief Case promos and the semi-Trophy card The Masked Royal), the Lady full art from Forbidden Light isn’t released in English, and at this point I doubt it will be considering the English Forbidden Light set was six sets ago…
I think we will see most unreleased Japanese SM era cards in Hidden Fates, the set after that, or promos perhaps, but I doubt we’ll see all of them in English. Unfortunately I can’t predict the future, because I would love to know if and when some of the Japanese FA Supporters or Pikachu cards will be released in English as well. 
EDIT: it was Shiny Glaceon actually, and they’ve also already revealed these Shiny Zoroark and Metagross boxes; these Charizard/Gyarados/Raichu tins that were from the Japanese Family box; and here they also mention a Hidden Fates Pin Collection featuring Mew and Mewtwo (no actual pictures of the product/cards yet), so perhaps that one will contain the alternate art you were asking about?
I suggest keeping a close eye on PokeBeach for the latest news. 
Greetz,
Quuador