My theory.

[Scootashy 10]

I believe that with Twilight's new powers she will have to make a big decision risking her life and her powers. My last theory is that Twilight will control the stars, her cutie mark are starts,  Celestia's is a sun, and Luna's is the moon. Who else agrees? 

[True Rarity 1,239]

I also believe she will have to make a big decision.

But it will probably include her risking her princessdom for her friends.

BUT if she were to stay a princess I agree that she would control the stars. Good theory! 

[Mint_Melody 91]

I believe Twilight will control magic or at least be responsible for creating many types of new magic. I think her cutie mark is supposed to be quite literal. A 'spark'. Like a spark of electricity. I think she'll be like the glue that holds the kingdom in place. She is not light like Celestia, she is not dark like Luna, I think she will unite the two concepts in a way and be like the middle ground princess.

I've also wondered if Celestia and Luna (and this horrifies me to think of too much ) will transcend or move on to pony heaven or another realm where they will no longer physically embody the same realm, leaving the country to Twilight and Cadence. I highly doubt this one would ever be true, but it was a thought.

[True Rarity 1,239]

I believe Twilight will control magic or at least be responsible for creating many types of new magic. I think her cutie mark is supposed to be quite literal. A 'spark'. Like a spark of electricity. I think she'll be like the glue that holds the kingdom in place. She is not light like Celestia, she is not dark like Luna, I think she will unite the two concepts in a way and be like the middle ground princess.

 

I've also wondered if Celestia and Luna (and this horrifies me to think of too much ) will transcend or move on to pony heaven or another realm where they will no longer physically embody the same realm, leaving the country to Twilight and Cadence. I highly doubt this one would ever be true, but it was a thought.

That is a scary thought.

Even if Twilight stays a princess (and I'd be uncomfortable about that at first), I would be able to get over it maybe by the end of Season 4.

Her princesshood is changing the show.

But if Celestia and Luna go away, the show would take a turn so sharp, I wouldn't be able to trust the writers like I'm trying to trust them now.

[Pony Of The Month 534]

If Twilight wasn't controlling the stars before she became a princess, than who was? The unicorns? And if so, what do they do with the stars?

[True Rarity 1,239]

If Twilight wasn't controlling the stars before she became a princess, than who was? The unicorns? And if so, what do they do with the stars?

I don't know if the stars were "controlled" at all.

They come out at dark just like in real life.

When Celestia sets the sun, the stars can show, paired with Luna's beautiful moon.

Oh, and if they didn't come out themselves, Luna probably controlled them. Her mane and tail do resemble a nighttime sky with stars.

[Nature's Spell 526]

Luna controls the stars. I've always felt she will be the Princess of Frienship (Magic). Celestia rules the sun and is the defecto leader i.e. first among equals. Luna the night and dreams. Cadence love and the Cryatal Empire. Twilight Friendship and Magic.

[WishingtotheStars 15]

This isn't really a theory, but I remember Princess Celestia asked the rest of the ponies to write to her about friendship. And that is what Twilight Sparkle did, and she was trained into (become) a princess. So maybe Princess Celestia is training the rest of them to become princesses without them knowing...

[Acoustic Cloud 2,202]

I disagree on the stars part 100%
You are telling me that a powerful sun goddess can control 1 sun... but a new-found-princess can control 10 sextillion stars? I don't think so...