Based on the idea that, post "Filli Vanilli," Fluttershy should absolutely serenade whichever pony/ponies who've captured her heart ...
So let us assume that Fluttershy is in love with Rarity. And so she decides to surprise Rarity with a serenade, because she knows Rarity enjoys her singing and also that Rarity would love something romantic like that. But ‘Shy knows she’ll never have enough courage to sing solo, and she can’t ask the other Ponytones for help for fear of Rarity finding out about the surprise.
What to do, what to do?
Fluttershy decides to ask assistance from two of her most flamboyant and spotlight-loving friends–Pinkie Pie and Rainbow Dash. Pinkie is immediately game and in love with the idea. Rainbow thinks it’s cheesy and not at all cool but, ultimately, she’s happy to help out a pal. For a week, they rehearse the song Fluttershy’s chosen. Pinkie’s a natural–no surprise, given how much she loves singing–while Dash takes a bit of work to really get in-key, but soon enough, the three of them have the song down pat.
Rainbow Dash preps the choreography. Pinkie nabs DJ P0N-3 to do the back-up music. Fluttershy paces her cottage a lot and asks Angel Bunny if she’s making a huge mistake.
At next Saturday’s party at Sugarcube Corner, Pinkie and Dash hover near Fluttershy for most of the night, waiting for their cue. Finally it’s time. The lights dim, the music turns up, and the crowd hushes.
Fluttershy misses the intro by just a beat, as the sweat pours down, but quickly catches up. It helps that Dash and Pinkie are right beside her, smiling, encouraging, and then … and then Fluttershy’s eyes meet Rarity’s across the room, and everything else fades away. There’s only her and Rarity, and Rarity’s staring straight at her, eyes wide in wonder and amazement and something else. Something more.
Fluttershy normally hates being stared at, but right here and right now, she wants Rarity to keep looking at her like that forever.
Somewhere halfway through the song, Rarity and Fluttershy start kissing, and nopony at the party can quite tell who kissed who first, though it probably doesn’t ultimately matter. Rainbow Dash shoots a glance towards Pinkie, who goes right on singing, taking up the lead vocals now that 'Shy’s otherwise occupied. Dash just shrugs and keeps doing the harmonies.
Rainbow Dash has always been good at improvising–at winging it, as it were–and so she sidles up to Applejack while finishing up the song. Might as well get some extra bang for the buck here. Applejack snorts and rolls her eyes as Dash throws a wing around the cowpony’s shoulders, but there’s a grin on her lips all the same.
“You tryin’ to tell me something?” Applejack asks, her eyes sparkling.
“I’ve been trying to tell you for months,” Dash replies, with a grin of her own, “but you’re just too dumb and stubborn to listen.”
It’s left to Pinkie to take over the lead vocals, now, and utterly unperturbed, she finishes up the song solo and takes a big, deep bow at the end. The lights go back up, the music goes back down, and the crowd hollers and cheers. It’s more for Fluttershy and Rarity than anything else, but if Pinkie realizes that, she doesn’t let it show. Instead, beaming widely, as happy as she’s ever been, she bounces over to the refreshments table and pours herself a glass of punch.
Twilight is standing nearby, frowning thoughtfully at Pinkie as though she’s never seen the other mare before in her entire life. Pinkie tilts her head.
“What’s wrong, Twi? Didn’t you like our song?”
“Oh, no, no. It’s not that.” Twilight’s eyes quickly cut over to the corner where Fluttershy and Rarity have holed up. She clears her throat. “I just, uh … I don’t think I’ve heard you sing quite like … that.”
“Well, duh! That’s 'cause we’ve never sang a love song before!”
Twilight nods. “Well, you did a very nice job.”
“Thanks! And y'know? If you wanna hear me sing love songs some more, you just gotta ask!” Pinkie grins, and it’s not a grin Twilight can remember seeing before. “So! Do you wanna hear some more?”
Twilight blushes fiercely, but she doesn’t say no.