Definitely has potential to be a very fun game. Some suggestions I have that I'm sure others may have made at some point for the whole game;
1.) A way to tell how much EXP. you're getting so you know how close you are to Leveling up.
2.) Perhaps besides battling, a way to play as well. Perhaps mini-games or some kind of activity. Less EXP, but perhaps more money to earn or something.
3.) An expanded move set. Of course, the strongest move could be the Sonic Rainboom, but others like the Buccaneer Blaze, and then stat-inducing stuff.
4.) It'd also be fun if you could get more than 1 of the Ponies =) Princess Celestia and/or Princess Luna as "Legendary Ponies"! lol