Instead of adding Megas (which either made older Pokemon too OP or didn't fix them enough), what Game Freak should've done is scrapped Megas and instead update older Pokemon's stats, abilities, types, movepools, ect to newer Pokemon standards. It really isn't that hard to do + the designs stay the same and are not ruined by Megas

In addition, X and Y should've been the first games in the series to introduce regular patch updates. Meaning certain Pokemon, moves, abilities, types, ect could be buffed or nerfed when they're too over/underpowered