My theory on what happened to AJ. After she disappeared, some say she turned into a crystal pony, without a reason? Before, I say what it is I will cover a little about crystal ponies. I think crystal ponies is an earth pony that wants to become a different type after eating the berry. There can be crystal ponies that stay the same for if they wanted to stay that way. Though maybe if they get corrupted by not making up there they become changelings. Now with that little details out of the way, you are wondering why am I'm bringing this up. Well you see while yes she becomes a crystal pony, she also becomes a different kind of pony. Your mind blown yet? I think AJ will become a pegasus! Why...

This picture->
Rainbow Dash and Lightning Dust flying S3E07

Or maybe I'm over-thinking it, maybe not. They both are smiling. They might not be friends not rival. Who tries to compete with RD? That would be AJ. Of course I might be over-thanking. It could also her mother or maybe she does have a sister. Who knows.