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I decided to create a Team for each important Kalos Trainer.
I made the Flare Admins as type specialists for the missing Types (Dark, Ground, Flying, Ghost, Poison and Normal) and added a Mega for each Trainer.
Since it's Generation 6 and each Full Team has 6 Pokemon I decided to do, as much as possible, the teams to have Pokemon from each Region (with the Pokemon besides Mega Blaziken and Mega Latios) being part of Kalos Dex.
I also created some Teams for the Battle Chatelaine based on a Mega Starter (sadily had to use Mega Venusaur for Ramos as it's the only Mega Grass Pokemon besides Abomasnow which is the only Ice Mega in XY) and gave them either one of the Legendary Birds (which are in Kalos Dex) or Dragonite (which is also Part Flying and has quite similar BST). Since all other Kalos are already used I gave each 2 Kanto Pokemon.
I also created Teams for the Rivals with Tierno still being around Pokemon with Dance moves (at least one dance move per Pokemon), made Trevor a smart trainer resolving around Trick Room (maybe also make battles with him Inverse Battles since it was an awesome idea hardly used) and Shaina being a Dog Lover. Emma in her Essentia Form would be a cat lover which replaced her Espurr with Mega Mewtwo for the Battle (seems like a really nice Boss Battle, Mewtwo form differs per Battle) while Sycamore as a Mega evolution specialist uses a Mega Latios (or Latias depending on the Version) which is in the Game code but not in the Kalos dex and Pokemon which got Megas in Omega Ruby and Alpha Sapphire (as a nice Easter Egg). The Team with Calem is, let's say, the Player Character Team. I would say all of these characters could've been battle at the Battle Chateau.