"Pokemon that are marketable and beloved (so no Seaking-GX, sadly)."
That right there automatically kills the discussion for me. While some of my Pokemon choices might be easier, such as Raichu-GX, Alolan Raichu-GX or Electivire-GX.
The "marketable" may not apply to 2 of my other favorite Pokemon, Honchkrow and Galvantula.
Of course it's impossible to make GXs out of other beloved Pokemon of mine. specifically Ivysaur, Wartortle, Porygon2 and Murkrow, but that I'm fine with since GXs, Lv Xs, and EXs have always been completed forms of Pokemon extended further.
Then you have people who want to see Alakazam return, what do you do about that? Well considering GXs evolve from prior stages, it's highly unlikely the Pokemon company is going to get over their Geller-phobia and release a Kadabra card, so their only option is to make yet another Abra that "Super evolves" into Alakazam-GX
Now what about those people who actually WANT a Seaking-GX? I can't say how much Guardians Rising is selling in comparison to Sun and Moon, or Evolutions, or Steam Siege. But the idea that pokemon HAVE to be marketable and beloved is nonsense. You honestly think players wanted a Wishiwashi-GX? No, players wanted a GOOD Wishiwashi-GX, but you can't always get what you want when it comes to ultra rares. Players and collectors will continue to buy packs and singles for the sake of collection and competition, and, call me crazy, but as a guy who has played Pokemon, YuGiOh, Magic the Gathering, Duel Masters, Weiss Schwarz, Power Rangers, Digimon, and Dragon Ball Z in TCG format, but I don't think it's unusual for trading card companies to make ultra rare cards that are utter trash -COUGHSLAPRASGXSTARCARDSCOUGH-
Can you honestly show me a single Ho-Oh card that has seen competitive play or is insanely expensive because it's a crystal card? Yet they keep printing them. Players WANT good ultra rares but that hasn't stopped Pokemon from making Lapras-GX, Wishiwashi-GX, or Bewear-GX. Some people love Wishiwashi and the idea of it as a full art Rainbow/Hyper rare probably made their day, regardless of the $4-5 value it holds.
Bottom line is, if you want advice on what Pokemon to make a GX out of, but not Pokemon like.... Seaking? Then it was incredibly stupid tp ask, because at that point you may have well have asked "Give me an answer I want".
Profitable Pokemon include Pikachu, the Starter lines, legendaries, pseudo legendaries, Snorlax, Eevees, Gengar, Machamp, Crobat, Lucario, and Aegislash.
They don't wanna hear "Scrafty-GX", or "Beartic-GX" or "Jynx-GX", they wanna hear "Dragonite-GX", "Salamence-GX", and "Togekiss-GX", and god forbid, ANOTHER Mewtwo or Charizard.
I apologize for the ranting, but that one line just absolutely drove me up a wall. If there's one thing I absolutely hate about the Pokemon community (That I feel the need to point out the Digimon community lacks) is the whole "My Charizard is better than your favorite" nonsense.