I'm personally hoping that Starlight will be treated on the same level as Spike and the Cutie Mark Crusaders.



Sunset seems more likely to be a part of the Mane Six anyways, considering how it's now abundantly clear that she's an Element of Harmony, much as how Twilight and her friends are Elements. Starlight...not sure. It's something we'd need to consult the Tree of Harmony about.

Plus, we can't have a rainbow with two purple colors! Sunset's red is a much better fit.



(On the other hand...it's very likely that Sunset is going to become an Alicorn, considering the many instances where she has had wings (Devil Sunset, "My Past Is Not Today", Daydream Shimmer). That's just my thoughts, though.)