some think they might be cooking up a special twilight masquerade doll, i doubt they're having trademark issues
other theories for this new girl include: new main character (mane 7), twilight sparkle relative (cousin), twilight sparkle replacement (a joke), new minor re-occurring character, twilight or luna's new student, and much more!
i doubt she'll be mane 7 for various reasons, replacement and twilight's student isn't going to happen, the others are plausible (i say re-occurring is more likely than luna's student)