Making Sense of Equestria Girls

With regards to worldbuilding that is.

First things first, I've recently seen a bunch of people suggesting CHS is a combined school because of the timeline. And it makes so much sense I can't believe I didn't think of it before.

Actually, FIRST things first...the Equestria Girls world shall be referred to as Pedestria unless otherwise noted. It wasn't my idea, but darn it I'm sharing that everywhere I can.

Parallel Worlds:
There is clearly some kind of destiny entanglement going on between the two universes. Parallel counterparts just make no kind of sense otherwise, and Destiny is definitely a force in Equestria. However, there is not total alignment. The versions of the characters in the two worlds are definitely similar, but at the same time they are their own characters. And not everything that happens in Pedestria has a counterpart in Equestria, or vice versa. Actually, a detailed study of the two universes should give a good idea about both the nature and limits of destiny. Assuming that they really are the same, there are some major takeaways even at first glance.

First, we see that destiny is primarily relational. By which it means that it first and foremost preserves how individuals relate to each other. An authority is still an authority, as we see with Princess/Principal Celestia, but the TYPE of authority matters less than who they have authority over. The same applies to other friendships.

Second, Destiny acts more in terms of potential than a railroad. In CHS, the main characters had the same potential to become fast friends. But when Sunset drove them apart, circumstances didn't conspire to drive them together again. While Sunset is a bit of an anomaly herself, this shouldn't be dismissed. Destiny ensures that the potential for that relationship exists, but what people do with that is up to them.

The weave of destiny is most likely tied to how ponies become younger when they travel through the portal. The magic thoroughly adapts them to their setting, and part of that is finding a best match for their role that preserves relations as much as possible. Twilight is an adult, but to destiny that matters far less than the fact that Celestia is an authority to her and her friends are her peers.

Portal Transformation

I think the best model for how it works is Connection from the Mistborn books. In short, someone in those stories can use Connection to subtly retcon themselves as if they had always lived in that place: instantly learning language and idiom and so on.

Similarly, the mirror transforms you as if you were a native to the world it takes you to. But that's more than just changing species. When Twilight arrives she doesn't need to spend months learning to walk, she just has to stop thinking too hard and spend a few minutes getting used to it. She speaks the language (which, since destiny is rather loose, probably isn't actually the same) and winds up with not only clothes, but culturally appropriate ones. That sort of thing.

What the Heck is Up with Sunset?

Specifically, why is she the only one we've seen with no counterpart? There are a few explanations.

First, it's possible that part of Sunset's destiny is to travel to other worlds. In which case, there may either be a formerly-human Sunset in Equestria OR human sunset may have found a portal to a completely different world. Hopefully when she returns to Crystal Prep it won't be to steal a fancy looking piece of Science to use to conquer her homeworld...

Secondly, it's possible that Sunset never had much of a destiny, and so had no counterpart. In which case, everything she does of significance is actually an anomaly. It's not something that destiny actually minds, as we've established it's flexible. But she is very much making her own destiny. I'm not quite sure I buy it, but it's possible.

The final main option I can think of is that she was formerly close to Principal Celestia, maybe her daughter (adopted or otherwise), and ran away. She hasn't returned yet.

History

There is one question that we have to ask. What's with all the horse puns? It's a human world! Should we just blame destiny again?

No. In fact, Pedestria originated from a culture similar to the Rohirrim, or the Mongols. Horses were incredibly important to them. And, like Equestrians, their descendants loved puns.

Celestia and Luna were totally involved in all sorts of adventures in their younger days, before they went into education.

Cinch and Sci-Twi

We've mentioned that Destiny is loose, and that events can be bent out of shape. Normally this doesn't happen. Principle Cinch is an exception, taking over Crystal Prep. This had a number of knock-on effects, and one of those is that is that Twilight's friendless phase was as a lonely gifted student at CP.

Sci-Twi and Sunset with Wings?

Princess Twilight manifests wings and a horn in Pedestria when fully powered up. That makes sense, she's an alicorn. Rainbow and Fluttershy manifest wings, suggesting that their counterparts being pegassi matters. But then it gets weird. Sci-twi ALSO manifests a horn and wings, and so does Sunset. And Sunset, at least, we know for a fact isn't an Alicorn in Equesetria. So what gives?

Short version, the Wings-And-Horn is a completely separate magical distinction that Princess Twilight already happened to qualify for. That she's also an alicorn in Equestria is a coincidence. Most likely it represents a certain threshold of magical power rather than all 'unicorns' having wings, since Rarity doesn't do that. It's certainly not harmonic attunement either, since Midnight Sparkle definitely doesn't qualify.

Equestria Girls Magic

First of all, it's obviously a world with bardic magic. Powering up from songs is proof enough of that. It's also a world where the Harmonic/Discordant magic dichotomy is also in effect just like Equestria. The main distinction is that it's much more accessible in Pedestria, and goes from 'headcanon' to just plain 'canon'.

Additionally, personal magic is clearly tied to someone's destiny. Sunset's ill fated experiments did at least show that magic tends to manifest in the form of the originator's cutie marks. This strongly suggests that personal magic is similar to a cutie mark talent. Of course, in this world those same marks manifest differently than they do in Equestria. It's fitting, as the humans are in fact different people than their pony counterparts.