Idea sounds good, but you should chose between an open world do-whatver-you-want kind of game and a linear game with a linear story. Combining them might work but for a project that is so small i think you should really just focus on just one thing and do that bit right. Now i would love a MLP open world game where you can go grow your character from a little filly to a full grown mare, making friends and spending lots of time building up your character, maybe even building your own town and taking care of it and all that fluff, so then you can share it with your friends and here rather than following a story and then just never play it again. I personally want to make such game, even though it might not be as big as i want it, probably 2D and in C++ without an engine.
Can't wait to hear more about it and i hope you're gonna be able to make something good out of it. I'm not an expert, but if you need help with debugging or anything C++ related problems i might be able to help you. And i would also love to be a beta tester for it :3