Twilight smelt fire. It was a smoky scent, quickly followed by shrieking. She shot up and opened her eyes. Smoke filled her nostrils and made her eyes water. After blinking, she saw Rarity's boutique, up in flames. The orange light danced across the front lawn, making shadows stretch and claw towards Twilight. Stumbling to her feet, she raced to the front door, then had a sudden pang of deja vu. This was the night Rarity had died.
Twilight already knew what happened. Using her magic to fling open the door, a wild inferno greeted the princess. Hysterical sobbing was merely background noise compared to the crackling, whistling voice of the flames, but she heard it. Rarity was laying, bawling on her couch. Memories flickered through Twilight's brain. All the times Rarity had seemed to pull it out of nowhere during one her, "episodes".
"Rarity!" Twilight called, "I'm here to save you!" The white unicorn, with tears glistening in her eyes, shook her head and let the flames swallow her up.
'No' Twi thought, 'I'm not losing you again!' and then bolted into the fire. It licked her flank and singed her, but she felt none of it. Upon reaching Rarity, the unicorn blinked up at her in surprise. Her once white fur was completely black with red patches of skin showing. Resisting a shiver, Twilight levitated Rarity up and ran out the door, the blaze hissing behind them. A loose board fell in front of them, but Twilight burst through the brittle wood, causing splinters to shower down.
"T-twilight!" Rarity stuttered as the boutique behind them fell apart and the beams cracked and cried out like wounded animals. Then it was silent. Not a sound was heard from the fire, and everything was turning a marshmallow white. Eventually, while the two ponies watched, it became a blank canvas.
"Rarity, I miss you so much." Twi bowed her head and recoiled, seeing that Rarity's cutie mark was glowing and all the wounds were healed.
"Thank you, for saving me. It could have been eternally that way, reliving the night I died, over and over. All because I left with regrets." A bolt of fear ran through Twilight's mind. Everyone else could be suffering?!
"There's something I have to tell you..." Tears sparkled in her eyes as Twilight helped Rarity to her feet. "Rarity... I" she burst into tears, "I loved every single thing you made and I wish I had never doubted you when you were making our gala dresses... I still feel guilty demanding so much of you." Twilight collapsed onto the white, now shining ground. Rarity put a comforting hoof around Twi's neck.
"Darling, it's alright, what a silly thing to feel regretful about." Rarity began to flicker in and out of view, her horn letting of a small glow. "Before I leave, I must tell you something." Rarity gave a motherly smile, "Make sure Sweetie Belle stays out of trouble. Ok?" Twilight nodded and tried to wrap Rarity's fading body in a hug, but she simply passed right through her. "Thank you, Twilight, for everything." and then she let out a soft sigh, and blew away, like dust in a storm. Newfound sadness gripped Twilight's heart, like she had just lost Rarity all over again. Then a flash of light blinded her.
Slowly raising her head, she realized she was back in the graveyard. The rain still tumbled down in sheets. A small glint captured her attention. On Rarity's headstone, was the purple diamond Element of Harmony. The crack in it was healed, and it let out a faint glow. Reaching out to it, it faded. Forcing herself not to cry any longer, she averted her gaze to Pinkie's grave, placed a shaky hoof on it, and felt like her head was going to pop. Darkness enveloped her once more.
Frankly, I think it would be an interesting idea...
It's basically assumed that the other members of the Mane Six will die in their sleep... but well, they do happen to be the Elements of Harmony and they have a bullseye on them when it comes to bad guys. And well, though narrative necessity in canon means no bad guy will ever "get lucky" or manage to get the best of them in a lethal manner, it's a distinct possibility. Likewise, accidents do happen (like one's house catching on fire)...
Living with the sorrow of one's loved ones passing away is one. Living with guilt from their deaths due to failure is another matter entirely.
I'm also thinking it could be a final lesson of sorts ; all friendships end. Even if the bond is too strong for arguments, philosophical differences, ideological differences, and other conflicts to break it, friendship will be ended by death itself. Sorrow is a part of life, even mortal life. It is time for Twilight to transcend the need for their physical presence and accept the memories and growth they gave to her.
Apart from Twilight's own growth, I think the other Mane Six themselves also to accept. Just as Twilight's immortality and continuing life separated her from them, so did their own path and destiny. They, too, must accept that they have whatever comes next to deal with ; Twilight's place in in Equestria and theirs isn't anymore.
That was very nice, please keep going.