Meh, all bias aside, I suppose it'd be interesting to see Dawn with a Leafeon.
So far, Dawn's team has been: Piplup, Buneary, Pachirisu, Mamoswine, and Ambipom.
Disagree with me if you must, but I don't like the fact that 3/5 of her team are just "cuddly, useless pokemon", that's only merit is either contests (In this case, Pachirisu is still a hair weaker than Ash's Pikachu, while Buneary's more often than not is used in Contests in the Visual portion), or in Piplup's case, being an annoying mascot. (Yep... Soon one of the main characters will use Eevee, spam that, and whatnot.)
Now, Leafeon obviously would become a Contest superstar, according to Dawn's training style (Thank goodness Mamoswine makes a cultural imbalance), and well, become less "cool" than it would in the hands of another trainer. But, at least Dawn has something evolved that's not the cool Ambipom or the well, "Charizard clone" Mamoswine.
Now, with the bias back in, Dawn would probably encounter the Leafeon in the Battle Zone, since the Battle Zone looks to be a warm, summer-esque place anyway.