I think these would have to be my top five Pokémon that I think should get a GX.
5. Gengar. Gengar is a very popular Pokémon that does get quite a bit of cards in the trading card game (Not to the level of Mewtwo or Charizard though.) In the TCG though, his cards are usually very cool and interesting having effects that most other Pokémon don't have. Gengar would allow for a really interesting GX attack, and is just the kind of Pokémon that deserves a GX.
4. Scizor. I'm picking this mostly because I think Scizor may have some increased popularity do to him being playable in Pokken Tournament Deluxe, besides that though, he's a metal type, and metal really needs a buff in power, with their only really viable deck at the moment being Metagross GX.
3. Arcanine. Arcanine always seemed like a Pokémon that was loved by a lot of fans, but for some reason never got an EX. A GX would make up for him never getting one. He's a pretty memorable a popular Pokémon and is even from the first Generation. He could very easily be a highly collectable card, and an Ultra Rare Arcanine would be amazing. That mixed with the fact that some very cool art could be made with the little flame dog.
2. Weavile. Weavile just seems like he could be a really interesting GX. He'd have low HP, but he'd probably be a low energy costing, free retreating card, that could be one of the more interesting stage 1 GX, and he never got an EX, besides being a fairly popular Pokémon.
1. Empoleon. I think that some more starter Pokémon deserve a GX, but the one I'd want to see most of all is Empoleon. To few starters got their own EX. Empoleon is one of my all time favorite starters, and his inclusion Pokken tournament DX might spark popularity for him. It might even be cool if they changed his typing to steel instead of his normal water type, this is largely because I'd like to see more metal decks.