Were Sunset, Twilight, and Starlight meant to be the next Celestia, Luna, and Cadence? (In that order...)
Sorry, this is kind of long...
If you can't tell by my username, I love Twilight Sparkle, so I enjoy how much screen time she got in the later seasons even after finding Sunset and Starlight, but this theory has been taking over my mind, so I just had to ask.
I want to start this by saying I'm rewatching MLP to get a refresher. While I have many spoilers about how the plot goes, I'm only on late S7. So take almost everything I say with a grain of salt.
It's in her name "twilight," which refers to the end of night or the beginning of morning, putting her somewhere in the 5:00 AM- 7:00 AM range (I swear this is important for later). That aside, she took too much of an MC role only to be put in a Luna position at the end of the show. My theory is that it was going to be canon and that's why Sunset and Starlight were introduced, but their stories always seemed unfinished or CUT.
I haven't finished watching the Equestria Girls movies, so I'm not quite as educated on her, but I do know her basic character arc. It's also in her name, like Twilight's, so "Sunset" refers to the end of the day (are we seeing the pattern?) and falls between 5:00 PM - 7:00 PM. (My question is: how do you think she got her alicorn wings? I personally think it was when she stopped sci twi... but again I haven't finished the Equestria Girls movie).
How they relate to Celestia and Luna:
Let's start with Celestia and Luna. It's clear they're complete opposites, and Celestia takes a more dominant role, as her name isn't just related to the sun (as Luna's is to the moon) but to all of the cosmos. Also, we can link THEIR "times": Celestia is the princess of the sun, and I think of her as being around 12:00 PM, or high noon. Whereas Luna MUST be 12:00 AM or midnight. If we compare these times to twilight and sunset times, we see that C&L are complete opposites while T&S are closer and share a common time where both the sun and moon are present. I think this would affect their leading style and wouldn't be as separate, like the current princesses.
Starlight fits perfectly with Twilight and Sunset (maybe even more than Cadence did to Celestia and Luna) as their first names represent the sun, moon, and stars. (but again I wouldn't know how she became an alicorn, so if you have any ideas PLEASE tell meeee)
While Sunsets and Twilights are more easily identifiable, Starlight is where it gets a bit trickier for me. Cadence (to me) is the element of love, indicated by many love themes and her cutie mark being a heart. So we know Starlight's "element" should be something arbitrary and meaningful like friendship, but that's Twilight's. (or is it...). If I'm going off of the theory that this princess idea was cut very early on, then maybe Twilight wasn't meant to be the princess of friendship. (That's a bit far-fetched, so I have a better theory). Twilight's element of "magic" doesn't really make sense compared to her friends in the early parts of the show. I think she represents more of an organisation or harmony than anything and brings her friends together. In the Winter Wrap Up, she takes more of an organisational role and thrives. Also, in the theme song, her line is "and magic makes it all complete," as if magic is what combines or ORGANISES her and her friends. I'm a bit more lost on Starlight and Sunset, though, and I won't claim anything for them.