I have two;
A third Team Rocket set.
A ‘Evolutions’ style re-print/fashion single set that is based off of all four Neo sets, with full art shinings and maybe a few new ones like Shining Blastoise and perhaps the inclusion of Shining Mew/Fan club Magikarp too.
God I would buy the shit out of these sets.
I don’t think any new set would satisfy my fantasy only because I’m not a fan of the design of modern cards in general - but I would love a better callback to gen 3 (Primal Clash was about the closest I think I’ll ever get). Celestial Storm just copied and pasted artworks from that era, which I thought was kinda tacky. I would have loved to see certain Pokemon in old ex design, like Houndoom and Lanturn.
Side note: I really wish they’d bring back smaller sets (<100 cards). They’re so much more manageable and interesting to complete, compared to the string of massive sets with dozens of secret rares we keep getting nowadays.
I like to imagine what sets would look like with different artists. Take base set for example and have Himeno, Nishida and Komiya doing their own original art for each of the same card. I would definitely buy that set.
Probably an unpopular opinion, but I want 1st edition stamps to come back for all sets 
Not a set necessarily but I would love to see some iconic artworks reprinted as VMaxes or something. Like take the artwork from Neo Lugia, make it cover half the card or more, and make it a secret rare. Imagine the thrill of pulling cards like GS Ray, Base Charizard, etc but the art just covers way more area.
Any set where the rare card is actually rare
Pokemon company hires back Wizards of the Coast and bring back 1st edition cards. Using better printers this time with better centering and no print lines. (Go back to their roots) Even if it’s new Pokemon. Or they could do the other generations printed by Nintendo or later. But that will probably never happen and it would almost be weird going from full arts and rainbow rares back to Wotc style 1st editions 
I think that is pretty popular of an idea.
I would love a reprint of team rocket returns.
They’ve brought back shining pokemon. It would be awesome big that had retro gold star sets with The legendary birds, Tyranitar and other gen 4+ pokemon.
My other bit fantasy set for pokemon would be like what they do for sports and comic book TCG. They have parallels with numbered production. Precious metals gems specially. Imagine the frenzy for a 1/1 Charizard parallel haha. Also they had artist cards/sketches. Occasionally you could get a 2-4 card pannel drawing that is huge. Sets like this would appeal to collectors.
Not a set, but I wish they brought back point cards like the Fan Club cards. The Play! Pokemon club still exists (even internationally) and you can still acquire points.
Jamboree
Another trainers style set like gym series except with characters from the Pokémon universe not just battlers (stuff like Professor Oak’s Bulbasaur, Nurse Joy’s Chansey, Officer Jenny’s Growlithe)
At this point, just any sort of set where the theme isn’t, ‘these two types are the focus, they will receive marginally more support in the TCG and we will colour the packs accordingly’.
Bring back sets centered around places, landmarks, evil teams, or literally any other theme with a lick of nuance.
Southern Island 2. Make the background pokemon into their own cards. For example Pikachu, Meowth and Poliwhirl. That way you can see the original cards from different perspectives.
Also a Pokémon snap full 64 card set. Holo Charizard, SR Mew.
I’d like to see cards with anime specific variants like crystal onix or snowman snorlax
www.thegamer.com/pokemon-anime-exclusive-want-list/
bulbapedia.bulbagarden.net/wiki/List_of_alternately_colored_Pok%C3%A9mon_in_the_anime
Here’s another idea. Any set where more than 1 or 2 packs out of the booster box of 36 actually has cards worth opening… We thought it was bad that 2 out of 3 packs were trash. Now look. 34-35 of every 36 packs contain nothing. What a $0.50 holo if your lucky? A plain gx? Full arts. Gold cards. And rainbow rares are very hard to pull which makes opening boxes a waste of time
Nothing in particular, but general themes:
Pokemon as a part of a larger setting. I like when you have a lot of Pokemon in a specific location from different angles, and it feels like the cards are part of a larger picture. I really like cards where Pokemon are inhabiting an actual space as opposed to some void.
Change how the rarity system works. It feels really unfortunate that regular holos are basically bulk, and that every set has 40 “secret rares”, basically giving 3 prints to every GX. Even the normal GXs feel like bulk.
I’d like to see them more focused on making a few really nice “hyper rare” esque cards, and then make them actually difficult to pull, like they did with the shinings.
I really liked the concept of trainer’s Pokemon like in gym and rocket. Gold’s typhlosion, Oak’s Celebi, Leon’s Chareezord, etc. Maybe have the actual trainer in the artwork as well. I liked how cosmic eclipse did this with Erika and the Vileplume, as well as the full art basic Pokemon with the trainers. It was really nice.
That’s the point i was just making. GX’s and holo’s are bulk. You buy a booster box knowing only 1 or 2 of the packs are actually worth opening… Come on now your making people open 34 plus packs of trash per booster box? Really?
