Okay, I am going to rattle off a ton of Pokémon that literally have had barely any cards printed compared to those to date.
Giraferig, Stantler, All baby pokemon, Swalot, Breloom, Zangoose, Arbok, Kingler, Hitmonlee, Hitmontop, Furret, Linoone, Bellosom, Ambipom, Qwilfish, Mantine, Miltank, Slaking, Grumpig, Cacturne, Whiscash, Crawdaunt, Cradily, Armaldo, Tropius, Chimecho, Gorebyss, Relicanth, Luvdisk, Krikitune, Roserade, Rampardos, Bastiodon,Cherrim, Gastrodon, Skuntank, Purugly, Spiritomb, Carnivine, Hippowdon, Drapion, Rotom, Phione
Liepard, Swoobat, Conkeldurr, Gigalith, Leavanny , Scolipede, Darmanitan, Crustle, Scraffty, Swanna, Sawsbuck
Escavalier, Beartic, Mienshao, Volcarona
This list includes cards that have either not had a print in a while, Had few prints compared to other cards in the format, Pokémon that have not seen competitively play, could have potential in a competitive scene, Have not seen an ex print at all or in a decade, may not be known by fans and thus bring more love to pokemon that otherwise go unnoticed. Most of these are evolution lines and would fit nicely in the slower format we currently have. They also vary in type. I relized while looking through the list that to cover all types, cards have to be reprinted or the same pokemon is printed multiple times in less than 10 sets. However there are enough pokemon out there in which pokemon could still rotate pokemon and not print the same ones in almost every set (mewtwo). yes mewtwo brings revenue because its well known, but its pokemon. You could put a pokemon on a pack that isn't a gx in the set and people will still buy them. If I saw a pack arts of liepard, beartic, hitmonlee, and giraferig, I'm sure they would look pretty sweet just like most pack arts do. Marketing isn't hard when your product almost sells regardless. But if these pokemon aren't playable such as the multiple clefairy that have been printed, they will continue to be filler cards in sets and get no attention.