Eventually, I think GF will need megas for every single Elite Four in the Pokemon World. But I think there going after Gym Leaders as well.

This is what we have already.

Roxanne - Steelix, Brawly - Medicham, Wattson - Manectric, Flannery - Camerupt , Norman - Kangaskhan, Winona - Altaria, Tate & Liza - I think Gardevoir/Gallade would work. The only ones remaining are Wallace and Juan. That explains unpopular megas like Medicham.

Lets look at Sinnoh. Gardenia - Torterra when starters come, Fantina - Gengar, Maylene - Lucario, Crasher Wake - Gyarados, Byron - Steelix, Candice - Froslass when it comes in Z. That leaves Roark and Volkner.

Even Johto has quite a bit filled out. Falkner - Pidgeot, Bugsy - Scizor, Morty - Gengar, Jasmine - Steelix, Clair - Dragonite (Thats likely coming eventually) That leaves just Whitney, Chuck, and Pryce.

There isn't hardly anything for the Kanto gyms except Blue - Pidgeot, Sabrina - Alakazam, maybe Erika and Venusaur.