Twilight ruling? That would pretty much defeat the purpose of the whole "Friendship is Magic" theme of the show.

I've yet to read around about emerging theories but I already am not buying into this Twilight becoming a ruler stuff. That would make the rest of the show awkward or end the franchise early. BUT, I can see the finale being an episode of Twilight being prepped for it just for her to turn it down in the end.

But just to make sure, you're inferring Luna is jealous because Twilight is 'being prepped'  to be ruler, right? If that's the case, yes, that is a fun theory to the stink eye ;P. 

Not sure I buy into the prep-ping for rule idea, like Hat said it just doesn't feel like it would fit in. That and Celestia and Luna have proven they are effectively immortal and up until recently Celestia was ruling by herself so why would they suddenly need a third ruler now the second one has returned?
I also agree with the fact that Twilight can use dark magic readily is bad but what's potentially worse is Celestia's use of it. She seemed to be possessed slightly more by the dark aura thing than Twilight did which may end up being a story down the line. That or it they just decided to do it for this episode's storyline which would be kind of disappointing.

On a more immediate note, the theory floating around TVTropes is that Luna is going to be teaching Twilight next, explaining the book with Twilight's cutie-mark at the end of episode 2.

You'd think this thread would be booming now that the new season kicked off :/

What could Luna possibly teach Twilight? Patience? haha! Get it? Coz Luna was on the moon for a thousand years... er nevermind.

Just found this ad, it's pretty funny:

On the topic of activity, give it time.  There's not much to discuss at the moment.

Well they did just randomly toss out that Celestia and Twilight can use dark magic, and out of the three of them Luna would have the most experience with using it and trying to control it, so maybe that?

I personally really like the idea of Twilight starting up with a new mentor, gives her something to prove to a new teacher again and gives Luna more screen time which is always a plus in my eyes.  

We need more details. Next week is Too Many Pinkie Pies (and i finally saw that commercial where she makes the G1 face. The commercial basically runs with her saying "i couldn't possibly make a face crazier than this!"

Think about this.  Celestia is the princess of light and the sun, Luna is the princess of darkness and the moon. 

Twilight's color is, if you look, kind of a balance between Luna and Celestia (Luna's dark violet, Celestia's gentle, light pink makes her mid-range royal purple).  

Twilght's ability has been magic, something Luna and Celestia are the masters of, and what she brings to pretty much every conflict in Ponyville has been her organizational skills and her sense of order. 

Twilight as the princess of balace and order?  

It would also fit the toyline perfectly. What would play better among little girls than the myth arc of a plain "girl" becoming a Princess?