I'm going to only list my top 5 suggestions for GXs, because I have a lot more.
Simi-GXs. Well, I feel your looking for mons that'll sell out, they could at least do Simisear, Simisage, and Simipour-GXs, as they print those so often, but they're never good. These 3 have been been with us constantly since Black and White, and I think it's about time they got a little more attention. Not saying they'd be excellent cards, just these appaear so often they might as well have their chance to stand out. They're cute, well-known and deserve a little extra love!
Bug-type GX's. Mainly talkin Vivillon here, as it was widely known for having a large number of colorful patterns, while it didn't get much attention, I feel mainly the colors could help it stand out. A Vivillon-GX with say, 3 or 4 patterns on it put on it in a beautiful way would sell out so fast! People buy cute cards! However, besides Vivillon, we don't have many Bug GXs, or even many Bug type EXs for that matter. Pokémon Bug types can sell easily for their cuteness or just the plain look
Wailord-GX. Few Pokémon in the TCG have stood out as being the best I felt their kind, and while in the games, Blissey tips out the HP stat with 255, in the TCG, Wailord-EX has the the highest with 250 HP! Now, Evolved GXs tend to have more HP than Basic-GXs, so I'd imagine this would have a ton of HP, up to 300 even!
Arceus-GX. Still light in the topic of Standing out, we'll notice that certain types have their certain things to them. Grass has a lot of healing and poison power, fire can burn opponent's and do a lot of damage, metal tends to have ways to take less damage, etc. Now, there are now 2 mons which can be all of these types, being Arceus and Silvally. Arceus obviously stands out for being one of the most powerful Pokémon in existence, as well as being able to be any type thanks to its Multitype Ability. Now, and Arceus-GX would be cool, we'd all love that, however, I think it'd be a really good idea to make it kind of stand out in what it and only 1 other can do- be every type. (not at the same time, but you know what I mean) That said, I think some sort of Ability or attack, or even something like the Arceus cards in the Platinum sets I think, where they are all just different types, would stand out very well, especially to have attacks that, to kids, are "awesome". An Arceus-GX somehow being able to use all of the TCG types to its advantage would be a very god idea.
Milotic-GX. (And Gyarados-GX) Not sure if anyone has mentioned this one, but @Water Pokémon Master
said he'd only suggest it once someone else suggests it. And honestly, it's a great idea! Milotic (and especially Gyarados) have always been seen as Beautiful (and powerful) Pokémon, and they stand out very well to a ton of Pokémon fans! GX forms of these, especially of they had some sort of connection with each other in how the cards work, would not only be very cool but it'd be very popular among some Pokémon fans as well, especially those that have Milotic or Gyarados as their favorite Pokémon!