>Kanto
Brock - Mega Aerodactyl
Misty - Mega Blastoise(?)
Surge - needs a mega
Erika - Mega Venusaur
Koga - Mega Beedrill
Janine - Mega Beedrill
Sabrina - Mega Alakazam
Blaine - Mega Charizard Y(?)
Giovanni - Mega Kangaskhan
Lorelei - Mega Slowbro
Bruno - Mega Steelix(for Onix?), new Kanto fighting mega would be preferable
Agatha - Mega Gengar
Lance - Mega Gyarados
Green - Mega Pidgeot
Red - random between M-Venusaur, Charizard X or Blastoise

>Johto
Falkner - Mega Pidgeot
Bugsy - Mega Scizor
Whitney - needs a mega
Morty - Mega Gengar
Chuck - Mega Heracross(?)
Jasmine - Mega Steelix
Pryce - needs a mega
Clair - Mega Gyarados is fine, but a new one would be preferable, or M-Ampharos?
Will - Mega Slowbro
Karen - Mega Houndoom
Silver - needs mega Johto starters like Red

>Hoenn
Roxanne - needs a mega
Brawley - Mega Blaziken(?)
Wattson - Mega Manectric
Flannery - Mega Camerupt
Norman - needs a mega
Winona - Mega Altaria
Tate & Liza - needs dual megas, one could be Medicham though
Wallace - needs a mega, Swampert and Sharpedo aren't fabulous enough
Juan - see above
Sidney - Mega Absol
Phoebe - Mega Banette over Sableye
Glacia - Mega Glalie
Drake - Mega Salamence
Steven - Mega Metagross
Zinnia - Mega Salamence
Wally - Mega Gallade
Brendan/May - random mega Hoenn starter

>Sinnoh
Roark - needs a mega, Tyranitar might work though
Gardenia - needs a mega
Maylene - Mega Lucario
Crasher Wake - needs a mega, Gyarados would work though
Fantina - needs a mega, Gengar would work though
Byron - needs a mega, Steelix would work though
Candice - Mega Abomasnow
Volkner - needs a mega
Aaron - needs a mega, Scizor might work though
Bertha - needs a mega
Flint - Mega Lopunny, kek
Lucian - needs a mega, Alakazam would probably work though
Cynthia - Mega Garchomp
Cheryl - needs a mega
Riley - Mega Lucario
Mira - Mega Alakazam
Buck - needs a mega
Marley - needs a mega
Barry - random Sinnoh starter mega, or just Mega Heracross