She will debut Mega Evolutions for Kanto starters, calling it now!!!Okay I'm just gonna put this theory out there.
Because Serena looks a lot like Leaf (their hats!!!!), it may be possible that she's one of the trainers that got either the charmander/bulbasaur at Oak's lab the day Ash left for his journey.
NOW I KNOW THEY SUPPOSEDLY QUIT but there's no rule that says you can't unquit.
You never know.
*slinks away into the mist*