I wrote this up once before in another thread, sometime last year. Of course, emotionally I was right in the middle of the show closing, so surely that had a big influence on my ideas. I don't know if this is still the route I would take or not. FiM ended more peacefully, without such an emotional crisis. I guess I would have struggled just as much though. Some things just shouldn't end.
I don't think this would conflict with anything that happened in the EqT. This all began really playing on my mind after Forgotten Friendship kind of freaked me out. It had some dark underlying vibes in it that suggested a quickly coming event that was going to hurt more than anything. It was apparently their last summer together, they were working on the year book (a book of memories), and the memory stone was brought in as a plot device. Sunset crying out as her friends were yanked from her mind one at a time. That killed me. It still does.
This feels like a story that would be much more sobering than everything else in either dimension. Everyone knows what happens when high school ends. People who are friends get scattered like leaves in a hurricane, and slowly begin to lose touch. This would be a tragic thing in any universe, but in an offshoot of FiM, this is much, MUCH worse. These characters are echoes of a friendship in a parallel world that is integral to keeping the world safe. The friendship of the Mane 6 is part of a prophecy, and a very powerful being chose them because of how strong their bond is. If even a shred of that is reflected in EqG (and there's a TON of evidence which suggests it does), this isn't going to go well for the girls, and possibly everyone else.
Graduation would easily bring with it the most painful scenes in the entire series. I might as well point out that a lot of my thoughts about this are inspired by "The Muppets Take Manhattan." In what is probably the saddest thirty minutes of film in the entire Muppet universe, the characters who have been friends throughout all of college are forced to go their separate ways to find work. They stay in touch, but at best, they're lying to themselves that this is the process of life and how things need to be. But none of them are happy; they feel utterly lost without each other. Just think about it. Fluttershy with no emotional support, in a university full of strangers. Rainbow Dash playing her heart out supported by a sports scholarship, trying to ignore a nagging feeling that she abandoned her friends. Focus on studies is mostly what is keeping them sane, as they try to let go of the world that was Canterlot High and the friendship they had. This is hitting Sunset in a different way. Rarity is pursuing her fashion dream but she misses Applejack.
What's Sunset doing? Well, in Equestria she was potentially the greatest unicorn mage at the time, (probably) destined to do what Twilight ended up doing. In the human world, she at least knows well enough that music is a very effective way to channel magic. She also found her love of music through her friendship with the girls, so she's sticking to it. Part research project, part self-improvement, perhaps a career path, and also the best way she could imagine to honor her friends. She's still in the same city, not too far from Canterlot High. She's playing in dive bars for tips and is studying at a local music school. It's here where our story starts, with an almost love-sick acoustic ballad; the kind you almost only hear from independent artists in such places, a song that makes time stop. We don't need a backstory for how Sunset is taking things. We can figure it out as she channels her emotions into every chord and verse.
Sunset has already encountered a noticable problem. She hasn't ponied up since everyone departed. Comparing notes via social networks and phone calls it's revealed that neither have any of the others. Theories fly around about losing magic through growing up, a lack of proximity to the portal, and a bunch of other things. But moving away from the place of magic to the mundane, one might even begin to wonder if the whole thing was imagined, like a make-believe game, and none of the cool rainbow wizardry actually happened. The best thing they can think is to see what happens during the next summer, but life has already begun playing its terrible tricks. Money is tight, at least one has already been given a summer job as an intern at a local business, and there's a chance they all might not be coming home.
Sunset is sensing that the bond between the girls is beginning to fade, and she doesn't like it one bit. Being the outsider pony, this isolation is especially hard on her. She has more regular communication with Pony twilight now than she does the girls. She seriously begins to consider going back to Equestria, but her heart won't allow her to do it. It's not the most selfless thing she's ever done by a longshot, but she feels compelled to try and do something about this.
She does visit Equestria for advice though, and has a very long talk with Twilight, and the Sisters.
While Sunset is away, trouble emerges near Canterlot High. Something comparable to what Grogar could have been. It's been there since the beginning, sticking to the shadows. While other villains have foolishly tried to attack head-on, this one has been biding its time, waiting for the inevitable: Graduation. It knew time and the cruelty of life would eventually divide and conquer the girls on its own, and now its time to rise has come.
Emboldened by the talk, Sunset steps back through the portal with an idea. A way to get everyone back together before it's too late. ((and this is where this probably becomes a non-starter, because I'm sure nobody's parents would appreciate something this irresponsible happening in a kid's show)). All she can think of now is the Rainbooms. What if they were good... darned good? What if they were good enough to go all the way? Then they would never have to be apart. She proposes this over long-distance communication but none of them are convinced the idea could work. Sunset takes time off and scrounges up every last coin she has and begins traveling the country. Sometimes paying for it, sometimes getting free rides. She performs at places she visits to make some very-much-needed cash. She visits her friends away at college, trying her hardest to convince them that the Rainbooms have a real shot. Slowly she wins them over, and talks them into competing at an "American Idol" style reality show talent contest where the winner is given a multi-year contract and record deal. Deus Ex Machina, the contest is specifically for students, and no school wants to ever forbid their students from it.
Meanwhile back in the town of Canterlot High a battle of sorts occurs. The big baddie has made its presence known, and gets caught by the Dazzlings who attempt to fight it. They've slowly learned to use positive magic without the dark powers of their former selves, but they are no match for big bad. Defeated and drained of energy, the Dazzlings are given a choice. Either team up, "or else." It also promises to restore the amulets lost in 'Rainbow Rocks.' Once it discovers the Rainbooms are potentially going to be reunited, it decides to prevent this in the most obvious and cliche way imaginable. Time for another band to jump into the game show.
Eventually we end up at the finale of the TV show's season. Multiple songs from the Rainbooms, and an impressive newcomer, "The Apocalypse," consisting of the Dazzlings and a mysterious lead singer (this villain would absolutely have to be voiced by someone from a famous rock or metal band). A lot of corporate mudslinging, manipulative tactics worthy of a live-action drama surrounding entertainers, a "magic manifested" fight that the entire audience is convinced to be just some darned good special effects. This is followed up by an eventual battle between the six and the villains after the contest has been won. They're quickly backed up by Canterlot High alumni who came to witness this event live. The Dazzlings willfully destroy their amulets in an act of defiance, betraying the villain and joining the Rainbooms to stop what's left of the Apocalypse. With the insanity of what has just transpired on live television, ratings for the show went through the roof, and a prosperous future for the Rainbooms is all but guaranteed. Cue the happiest celebration imaginable, a marvelously sappy ballad about friendship lasting forever, oh and the Rainbooms may have picked up some additional members. Credits roll with newspaper clippings of where things go from there. The Rainbooms become a global sensation, spreading a message and love and friendship through music. They perform for impoverished cities, they play in warzones, and in front of national leaders, visit children's hospitals, and have their own charity. The Rainbooms have become the epitome of a force for good, changing the world for the better; having achieved a comparable level of influence as their pony counterparts.