They should make a Tier where you can only use all Pokemon of a certain colour group:
Some notable Pokemon in each colour group:
Red: Charizard, Scizor, Latias, Rotom, Bisharp, Talonflame
Blue: Gyarados, Azumarill, Altaria, Heracross, Metagross, Latios, Garchomp, Thundurus
Yellow: Beedrill...