@b_jonas beware - some spoilers may be very heavy. As I said, just having seen a picture of Songbird probably spoiled a mayor "plot twist" (if that could be called plot twist.... It is like Gaia Everfree... her design gave away her identity the second you saw her)
Up to now, the 2017 movie leaks have given no indication of Starlight even appearing in the movie. I am pretty convinced that the plot wasn't really build to have the "main 7" but just the old "main 6".
When Lauren drawn the basis for the show, she had a precise idea in mind. MLP wasn't going to be once again just a "girly show full of cute thing". Instead, she aimed at a show that was deliberately crafted to be the the exact opposite of all the teenager animated shows/sit-com that were very popular back then (and are still popular today)
^ Ah, this is what you mentioned again yesterday.
When you talked about why you don't like how they're treating Starlight in S6.
It wasn't bad just because he didn't had a backstory / development (which still would only help, and that is probably why he end up to be much more likeable in the last movie when they try to give him an actual character)
the point is that he was a love interest. And that pointed in the mainstream shows direction
I may have mentioned my favorite bad example: Jules Verne's Voyage au centre de la Terre. Most of the book is three people traveling underground, completely separated from other humans. The first quarter of the story is preparing for that journey. There's a pretty good reason why the three protagonists meet no other characters.
The good example is Les av. du capitaine Hatteras. It is about the journey of a ship to unknown lands, so there's no opportunity to meet anyone, especially not exotic females who could become a love interest. And captain Hatteras goes for sure: he hires a crew of 20 unmarried male sailors. Unmarried is specifically a job requirement he states.
@Derpy Flash Sentry at least has a minor payoff in the second movie, when Sunset admits "Flash is a great guy and all, but I never really liked-him liked him. I was just using him to become more popular." He's useful to tell something about Sunset's character. That's still not enough reason to make Twilight fancy him.
@Derpy What? Other animated shows aren't trying to sell toys or other extra stuff?