This will definitely be a brief section, but one very much worth mentioning. After trashing the Elite Four, head along into Lumiose City. On South Boulevard, there is a café, Café Soleil - it is the same one in which you and your rival first met Diantha, before you knew her as the Pokémon League Champion.
There, you can find Diantha again. Speak with her and you’ll learn that she wants you to trade her any Pokémon for her Ralts. Seems like a kinda crap trade for so late in the gate, right? WRONG! Well, sure, it’s a Level 5 Ralts, not like there’s anything special about that (even though they are a little tedious to find).
But, rather, it’s what the Ralts is holding - a Gardevoirite! Gardevoirite allows Gardevoir to Mega Evolve, similarly to how you saw Diantha Mega Evolve her Gardevoir when challenging the Elite Four. Ralts will evolve into Gardevoir at Level 30, which shouldn’t take too long at this late stage in the game.