Some I think would be pretty epic:
Ho-oh (holo, Japanese old-back promo, with Sugimori artwork as on the Gold cartridge)
Lugia (holo, same/Silver cartridge)
Rival (secret-rare Neo Genesis holo, with the artwork of the red-haired rival from the game, again by Sugimori)
Red and Blue as trainers in the VS series (with six 'mons each), maybe with Red’s drawn by Sugimori and Blue’s by Himeno (any suggestions of artists that aren’t already covered in VS?)
I’ve had this image in my head for a while of Pokemon releasing a super limited promotional print of PKMN Trainer Red and Blue full-art trainer cards. Probably the only trainer cards I’d be super hyped about.
I wish there was a Ash full art trainer or Red.
I have also always wanted more cards like “Ash’s greninja” just of all his pokemon.
Venusaur & Blastoise in XY Evolutions…
‘8 bit’ cards with artwork taken from red and blue in game sprites. Also a missingno card.
Team Rocket: Dark Venusaur
Gym Heroes/Challenge: Misty’s Blastoise, Rocket’s Articuno
Neo Destiny: Dark Meganium, Light Umbreon, Holo Light Vaporeon/Jolteon/Flareon
BW Block: SR Samurott, SR Venusaur, Suicune-EX FA, Regice-EX FA
This is good.
Tyranitar gold star
Ho-Oh and Lugia gold star
Suicune FA in dark explorers.
Moltres FA
Complete FA regi trio
These would have great in Evolutions, they’d have to be a bit more stylised though. These look too cartoony.
Or this as the next secret rare 
Plasma storm Venusaur SR shiny
Shining Gyarados Neo 4 style.
Deoxys Gold Star
Darkrai Delta Species Holo
I’d also love if we could see all the new GXs with actual good BW/XY FA artwork and not this SM solid-color garbage.
Any of the GB Cards in the Pokémon bible that they decided to make mockups for but couldnt be bothered to print 
We def need a Rocket’s Articuno and Dark Meganium!
I have wanted to see a Full Art Professor Oak.
That Magikarp salesman from the Pokecenter at Mt. Moon… lol
Snap Ho-Oh
Furiously researched to determine whether those existed after the first time I saw them. Maybe they “exist” like the Prerelease Raichu!
Lorelei. Most of the Johto characters (in English). Most of the Hoenn characters (in English), etc. It bugs me that so many Trainer cards are just made up nonsense that has nothing to do with anything when there’s a ton of content from the video games to mine for ideas.
Also, baby Pokemon after Call of Legends. I don’t like most babies, but these Pokemon exist and every Pokemon has fans, so none of them should go literal years (or a full decade) without having cards just because the card designers are too lazy to think of a creative way to make Baby evolutions viable without being broken. And then there’s Kadabra, but we know why in that case.
I wish the DPPt SH shinies had the same foil treatment as their Japanese counterparts instead of that generic, easily-scratched reverse foil.
Also wish they would use a random Pokemon generator to choose shinies for each set. I just generated six (Octillery, Herdier, Slakoth, Weezing, Staraptor, Togepi) and think this would be a really fun way to showcase 98% of the mons that get neglected.
Flygon EX FA and Salamence EX FA! Both of these Pokemon only got their ex-cards released on Promo boxes and there was no FAs.
Also the lack of Latias EX on the XY era was quite odd. The only Latias cards released were Promos as well.
Plus I agree with few earlier posts, all the missing parts of trios released on sets should exist. Imagine how beautiful a Suicune EX FA would look like! The lack of Venusaur and Blastoise holos on Evolutions is a sour grape too. Dragonair too! They left the most beautiful card of the Base Set away D: