After my Chandelure and Arceus suggestions on the third page, I thought of a few more ideas. Honestly, a lot of the reasons for what I want to see are simply from remembering previous cards and thinking about how much I liked them or wishing they had seen play because they were interesting or had potential.
Gengar GX could be neat. There have been so many Gengar cards that have been interesting. I think since it's typically depicted as a prankster, maybe making a card where you're essentially playing your own meta game could be fun. The biggest thing that comes to mind is Gengar Prime. I'm sure the Lost Zone was just a one-time thing, but it's so intriguing. An Ability where whenever an opponent's Pokemon is KO'd while Gengar's active, that Pokemon (but not the cards attached to it) are sent to the Lost Zone. Maybe an attack to spread damage or at least deal significant--but not heavy--damage to both the Active Pokemon and a benched Pokemon. A GX attack that sends a certain amount of cards in your opponent's hand to the Lost Zone. In a format that actually uses the discard pile as a mechanic and relies on it, something like this could be devastating. Another Gengar that comes to mind is the Poltergeist one. The only thought for mentioning this is because when you played that, your opponent had to play differently because of the consequences that could happen if you didn't (or couldn't) play smart against it. (So like the Tashalanche Garbodor, in a sense.)
Scizor GX because all Scizor cards tend to punish your opponent for playing certain types of cards, either by preventing damage to itself (usually from your opponent's Pokemon that have Special Energy attached) or hitting hard against the heavy hitters (like EX's and GX's). You need a strategy to get around Scizor. Or play Fire, I guess. Ha ha. I loved Scizor Prime when I first started going to tournaments, and even though it was only big for a hot second, I loved seeing Scizor EX and Mega Scizor EX become a legit deck recently.
Hydreigon GX and Volcarona GX. Paired these together because I can't help but notice that both of these Pokemon have either had really awesome Abilities or attacks, but they never saw any play (or very little play). This is just off the top of my head, but Hydreigon is classified as the Brutal Pokemon, so why not make it a heavy hitter while having an Ability that supports itself like making all energies attached to it Darkness energies or giving it a boost in damage based on something your opponent has in play? Maybe Volcarona could play off of the Burn status condition with an Ability, like burn inflicting more damage and/or your opponent can't retreat burned Pokemon. The attack would do however amount of damage, but would also inflict burn. And I've noticed a few Volcarona cards have awesome set up attacks. Maybe the GX attack would be along the lines of Ho-oh GX or Solgaleo GX or a balanced combination.
WPM mentioning Kabutops GX makes me think about the fossils from the Arceus set. I love those cards' synergy! If the fossil mechanic could be fixed in Sun and Moon, I would love to see that synergy return! Kabutops was the early heavy hitter while discarding fossil Trainer cards, so that could be the GX out of these three. Omastar was the late game heavy hitter after you had so many fossils in the discard pile. And Aerodactyl searched out the fossils from your deck. Even Kabuto and Omanyte had their uses! Though, the Kabuto and Omanyte from Majestic Dawn had helpful Poke-Powers, as well.
That's all I have. I wanted to come up with interesting ideas (even if they've already been used), rather than just listing Pokemon.
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