Friendship is Dragons/WMG

The DM is male.
He thought it would be a good campaign for a bunch of girls. And since everyone else is female, we have to incorporate a Brony analogue in there somewhere.

Spike's story about the origin of dragons in Equestria is a Chekhov's Gun.
...Or rather, the GM intends for it to be. However, given how the players hadn't paid enough attention to the opening monologue to realize the importance of the Elements of Harmony, the chances of that going as planned will be... rather slim.
 * Alternatively, when we finally get to the upcoming episode "Dragon Quest" in-comic, this is gonna be the longest setup in a webcomic ever since 8-Bit Theater's 4 white mage combo, at which point only Fluttershy will be able to tell it is time for the migration because she was paying attention to Spike's speech.

Twilight will assign some of the others the wrong Elements.
As in, the GM intended for some of the characters to have different Elements from Canon, and is surprised when Twilight names and assigns Elements he didn't expect to some of her teammates. For instance, he might have intended for Dash to have Honesty and Applejack Loyalty, or for Applejack to have Generosity due to her being so helpful and guiding the others.

"Magic" may not have been his original idea for the sixth Element's name, either. Everyone just ends up going with it because she's so obviously on a roll/into it that they don't have the heart to interrupt.

While G4 doesn't exist, the first three generations do. The GM is basing this world on what she thinks My Little Pony should be.
And her name is Lauren Faust.

The GM Nerfs the Apple Clan.
AJ's player originally intended for the whole clan to stay and work in Sweet Apple Acres, earning lots of money for her adventuring career. The GM decides this is too overpowered and cuts it down to just Big Mac, Granny Smith and Apple Bloom in "Applebuck Season".
 * Justification will be either:
 * Applejack's parents were killed by monsters, giving her a tragic backstory and reason for becoming an adventurer.
 * Big Macintosh and Apple Bloom will be AJ's husband and daughter rather than brother and sister, to parody all the incest clopfics.

Rainbow Dash's and Rarity's alignments will slowly but surely change to Chaotic Good and Good, respectively.
A special point in the Mane Six is that despite their flaws, all of then are good ponies who would never intentionally harm somepony, but we have two characters here who are openly evil(admittedly, Rarity did try to hide, but didn't work). However, through experience with their newfound friends, they will learn that there is more on it that just thenselves. Rarity is already starting to change, by doing a trully selfless act without thinking twice(as in canon, giving her tail to Steven Magnet), and Rainbow Dash is the element of Loyalty, while it COULD lead to a Even Evil Has Loved Ones situation, she will start to care too much for the ponies around her to actually stay evil.

This will also give small life lessons to their respective players, who will start giving deeper thought to their regular actions.
 * Impossible. The game is based off of 4E. There is no Chaotic Good and Neutral Good. Merely Good.
 * What I meant is ACT as if Chaotic Good/Neutral Good. I am aware the alignments work differently, but the actions stay the same regardless of what it says on the character sheet. jerk-ish and tough-acting Good strikes me as Chaotic Good actions, and Rarity's actions so far gives off the idea of Neutral Evil style of Evil, becoming Good would lead to anything but Lawful Good(she is a thief, afterall).
 * RD's player did make the "Even Evil Has Loved Ones" argument; however, this isn't completely Jossed. Rarity's player is established as a devoted Roleplayer who might wish to act out a gradual alignment change as Character Development, and Dash's could still make the shift over time despite herself.
 * Rarity has officially shifted to "Unaligned".

Rarity's player is (or will become) the Dungeonmaster's Girlfriend.
Spike's flirting with her is Foreshadowing. If the relationship already exists, the other players aren't aware of it yet, or mostly unaware, with them eventually figuring it out over time. Otherwise, the pair will get a Relationship Upgrade over the course of the series.

My Little Pony: Friendship is Magic do exist in the comic's universe, and part of the condition to take on the campain was to never had watch it before.
Exept Twilight's player did watch it before, she just haven't realized it because she just watched the last few minutes of the second episode, and could only remember it because she thought it was incredibly awesome, it only kicked in when Applejack was ariving into the scene, and she finally noticied the similarities.

She will mention it by the end of the session, but only to the other players, at which point they agree to keep it a secret, as a way to thank Twilight for that awesome end of session.

The Cutie Mark Crusaders will also be Player Characters.
This could be for a variety of reasons.
 * Not all of the regular players will be able to show up to every session, and the Cutie Mark Crusaders are created to try and avoid splitting the party.
 * Side-effect of Applejack creating such a big family: the GM expects her to do something to flesh them out more.
 * An attempt to draw Fluttershy's player out of her shell and make her a more confident player. If she plays one of the Crusaders (most likely Sweetie Belle), the events of "Stare Master" will be initally focused on teaching her Divergent Character Evolution.
 * Punishing Dash's player for all her complaining by illustrating how it could be worse: she could be a pegasus who can't even fly and has even less reason to go adventuring!

And the reason they keep Comically Missing the Point on how to get their cutie marks...? Their players don't want to get 'locked into' a special talent before inspiration hits!


 * In keeping with this, Twist, Diamond Tiara and Silver Spoon will be NPCs the GM designed not only to flesh out the game world and give them recurring characters to interact with, but to have premade characters in case the other players show interest in joining the Cutie Mark Crusaders' campaigns.

Who will play the CMC?
Based on the above theory, if the Cutie Mark Crusaders are not played by new arrivals to the campaign and are taken up by people we already know:


 * Apple Bloom: Twilight's player, trying to expand her repitoire and prove she's not completely reliant upon magic.
 * Sweetie Belle: Pinkie's player: given how Rarity reacts to her main, having the Spoony Bard play her little sister as well would be hilarious.
 * Fluttershy's player could also work, though "Stare Master" would require considerable rebalancing.
 * Scootaloo: RD's player, hoping for more action. She could also be attempting to groom Scoots as a potential backup character in case anything happens to Dash.

In all these cases, their 'main' characters showing up could be balanced by the DM temporarily playing them as a NPC, or handwaving their access to information, such as Twilight's book.
 * Jossed, they were in a guest comic.

The Cutie Mark Crusaders will be played by Applejack's(Apple Bloom), Rarity's(Sweetie Belle) and Rainbow Dash's(Scootaloo) players younger sisters.
It will all start in a day they have a session planned and one of then have to babysit her sister, so the other two offer to take their younger sisters with then so the kids would be busy with each other while they play, and they get a bit too interested in the game, so the GM gives off the idea of having then playing too, while AJ, Rarity and RD are against it, everyone else likes it and they play the session by introducing the CMC.

The reason Scootaloo isn't Rainbow Dash's sister in game is the same meta-reason we have in canon, the GM thought it would be too dangerous for a young filly be the sister of the Chaotic Evil barbarian, so they have her as Dash's biggest fangirl instead.
 * One of them could be a younger brother instead...
 * Little boys could be a little disgusted by the idea of playing with ponies, but could surely work for the tomboy-ish Scootaloo.

Owlowiscious will be Twilight's player attempting to get a new familiar.
Since the GM provides Spike's characterization, she'll get annoyed enough with his snark to try and replace him with a new familiar -- one who can't talk. The GM responds by attempting to lay one massive guilt trip on her.

Gilda and Trixie will be failed attempts at adding a Sixth Ranger...
...Both of which are played by another gamer with previous experience playing with Applejack, Dash and Rarity's players.

First, Dash takes a cue from how Applejack effectively 'bribed' the GM with a detailed character history by offering up the idea of Gilda as an old school friend. Though reluctant to incorporate a PC griffon, the GM agrees to give them a chance, especially since Dash seems to be roleplaying for once...! Unfortunately, Gilda's player is needlessly critical of the setting and characters, and gives Fluttershy's player grief for being a Noob, scaring her off.

Trixie, meanwhile, is a pure Min-Maxer who makes Twilight's player worry that she's falling into the same trap... or isn't optimized well enough compared to the other's expertise. She targets and humiliates Applejack, Dash and Rarity due to past grievances with their respective players. Her God Mode Sueing leads to the GM specifically targeting her.
 * Trixie will be optimized for PVP, but ends up completely outclassed when dealing with a large monster with massive damage reduction or excess HP from being a solo monster, and her skill set will be crap for the necessary skill challenge to soothe it.
 * Apparently not, she was Natalie's character in the Grand Line crossover.

"Look Before You Sleep" will see Rarity and Applejack's players arguing about the roleplay element.
With Rarity thinking that the other mun doesn't use the extensive Backstory she's created for AJ correctly and is treating the Apple Clan as mindless, shallowly characterized labor. AJ's player defends her decisions as practical and accuses Rarity's mun of overthinking things.
 * This could Foreshadow "Call of the Cutie", with the GM deciding to make AJ flesh out her family more by giving Apple Bloom A Day in The Limelight.

Discord will be the GM's way of giving Rainbow Dash's player exactly what they wanted.
Mainly, a serious threat. This will cause behind-the-scenes drama, with Dash's player getting so fed up she ditches the RP group entirely, knowing that will leave the setting scuttled without her Element to defeat Discord. This recontextualizes the big chase scene into a confrontation between the players as they force Dash to admit she's more attached to the campaign than she wants to admit.

Discord is the GM's attempt at 'rerailing' the campaign.
Though the changes he had to make in response to Twilight's taking the series Off the Rails work at first, the GM isn't completely satisifed. (If the series is kept entirely in order, he could be disappointed by the results of the Grand Galloping Gala... or, alternately, irritated by the players' reactions to it, or generally continuing complaints of the series not having enough 'action'.) The Return of Harmony arc, then, is meant to stick the group back on a road suspiciously similiar to his original plans, starting with De Powering the party.

Along the way, he reveals lingering frustration with how each of the players has conducted themselves thus far. In particular, Discord singling out Rainbow Dash for a far more sadistic version of a Leave Your Quest Test comes off as his attempt to ensure she doesn't handle it the same way as her first one. This leads to hurt feelings across the board, and the group seemingly on the verge of breaking up entirely.

On top of this, when Fluttershy's player doesn't go along with his plans for her, the GM actually, bringing her player to tears and making the others even more outraged and screaming that The GM Is a Cheating Bastard.

Their reactions eventually get the GM to realize he's become so focused on 'fixing' the campaign he's forgotten the all-important Element of FUN. This also foreshadows his loosening up enough to start giving the players more solo/semi-solo mini-campaigns, playing with the world they've created more.

Plus, there's something ironic about the Embodiment of Chaos being used in a heavyhanded attempt to force 'Order' upon the player party...

"The Last Roundup" sees AJ's player trying to put her on a bus.
She could have intended this as a temporary change, in order to try playing a different character concept for a while before bringing her back later, or a more permanent switch-up. Either way, the whole thing gets dragged Off the Rails when the rest of the group, instead of meeting her replacement, decide to go after her instead.

Daring Do's quest is a solo campaign for Dash's player.
Giving her an outlet for the sort of dungeon-crawling adventure she's more familiar with.
 * Instead of the conflict being Dash not wanting the others to know she's gotten into reading, perhaps her player's more concerned with keeping the others from discovering she's become so invested in the 'girly ponies' setting she's started playing another adventure in it.

For the Royal Wedding, Twilight will Metagame to figure out that something is affoot.
While the other players complain about the lack of action.