One little cramp later...
An afternoon stroll in the park. The birds feeding their young in nests high up in the trees and ponies lolling about without so much as a glance in her direction. Perfect. Celestia continued walking about looking at everypony who had been relaxing in the lowering sun's rays. She always liked walking out here when she had the free time and it wasn't an uncommon sight, so she didn't get much attention. As peaceful as it was, she had to remember what she came out here to do.
Celestia looked forward at the small hill that she was climbing before spreading her long wings and lifting high into the sky. It was only after she had climbed pretty far up that she froze and blurted out, "Wing cramp!" before tumbling back down towards the ground. Many ponies heard her and looked at the falling alicorn with a combination of terror and wonder at how somepony like her could get a cramp.
Only a moment later, pegasi from all over the park started flying, trying to catch her, but before any of them could get close, Celestia had hit the ground hard. A large crowd surrounded the area where she crashed as the dust began to settle and it was revealed that all that was left were some crumpled garments strewn about and a few feathers. Many were scared for what happened to their beloved princess and many more began to weep for the supposed loss of such a caring pony.
Celestia watched from far up in her bedroom at the group of sobbing ponies and felt a small pang of grief in her heart, but she also wondered, 'Had none of them seriously thought that I could survive a simple fall?'
Sometimes she wondered how Equestria has even lasted this long...
The next day, under the setting sun, a casket with Celestia's picture and bundles of flowers was lowered into it's grave. Everypony present was sobing and mourning the loss of their most beloved princess, but none more than Luna who had to be pried off the casket by both Cadence and the royal guards surrounding it. It had felt like after all this time, Luna was closer to her sister than ever before, then suddenly, she was taken away forever.
Celestia of course watched from far off in the hills with an enchanted cloak to hide her identity and a sorrowful heart. She hated to see her sister like this, but for the experiment to truly work, she needed to keep everypony in the dark. Celestia turned away once the funeral came to a close and walked off toward a place she knew she would be able to be alone, yet still close enough to watch Equestria closely.
"Excuse me, mam. But, why are ya in our treehouse?"
Startled, Celestia turned from her telescope to face the filly who had spoken, "...Spying," Celestia said carefully, "Spying on everything."
"Oh, cool! Can I join?" A second filly butted in. It took Celestia only a moment to recognize the two fillies as Applebloom and Scootaloo, "But, wait, who are you?"
"I'm... I'm..." Celestia stuggled for a psuedo name to go by, until an idea popped into her head, "Say, can you girls keep a secret?"
"Nope!" A third filly, who turned out to be Sweetie Belle, said from the doorway.
Celestia was taken aback, "Not even a really important one?"
"Nah, she's right," Applebloom interjected, "We always end up blabbing one way or the other."
"Fine, then call me Sunny Skies for all I care! I have way more important things to get to." Celestia grumbled out as she went back to staring at the castle through her telescope. The three fillies sat back and wondered why this strange mare was here, but ended up going back to planning for their cutie marks and paying her no mind.
Meanwhile, in Canterlot
"I can't believe she's really gone!" Luna wailed from her bed, with a conforting Cadence sitting next to her, "Sure, being the ruler of Equestria is pretty awesome, and having an army of guards and servants to do my every bidding, and having millions of ponies looking up to me... You know what? This might not be so bad!"
"What!? Your sister, my auntie, just died! You're telling me you're ok with that?" Cadence blurted out in shock.
"I'll let you rule the crystal empire!" Luna said in a sing-songy voice, "I heard it just showed back up a few days ago, and nopony has bothered with killing that Sombra fellow. You kill him, you can have the empire!"
Cadence continued to stare in shock until thinking about what having her own kingdom would be like, "You know what? Maybe you're right! That old coot has been around long enough, it's a surprise she didn't bite the dust until now!" She ended with a chuckle.
Luna joined in and told a servant to bring in some wine. Tonight, they would celebrate!
Celestia stared up at Luna's bedroom and saw colorful lights flickering across the curtains. She thought that it must be some form of attack from some angry unicorns blaming Luna for her death. Sure it had only been a day and the sun had just set, but Celestia knew that if her sister was in trouble, she would need to go help her. So, that's exactly what she did.
Celestia threw off her cloak and jumped out of the window into the sky as she flew toward Canterlot. The fillies, who were still in the treehouse, were having a sleepover. Scootaloo sat there grumpily as Celestia's cloak was drapped on her head before ripping it off and sneering at the currently laughing Applebloom and Sweetie Belle.
Luna and Cadence had gotten considerably drunk at this point and were dancing to some music under the quickly installed disco ball above. Suddenly Celestia bursts through the window knocking the music player over and breaking it. She looked around furiously for any hostile ponies, but only came face to face with her two family members. Her two terrified family members.
"I can explain." Celestia tried to say before a shriek erupted from Cadence.
"Zombie! Kill it! Kill it now!" Cadence yelled while grabbing a nearby table and breaking it on Celestia's head. Luna, in her drunken stuper, also grabbed for something to hit Celestia with. She found a lamp.
"OW! What in Equestria are you doing!? I'm not a zombie!" Celestia yelled, but to no avail. Luna and Cadence continued to grab anything they could to throw at Celestia. Wine bottles, serving trays, even the disco ball itself was ripped out of the ceiling to be thrown. Eventually, the two drunken mares ran away and left a battered Celestia to pass out on the floor.
The next day
Slowly, Celestia woke up on the floor where she had passed out. She thinks over how much better this plan was supposed to go, but instead of learning what Equestria would be like without her, she ended up beaten and bleeding on the floor.
Suddenly, a knock was heard from in front of her. The door was open, but the servant wanted to be polite. They walked in with some fresh breakfast and a newspaper before leaving. Celestia happily ate the meal as she read the paper until she realized what the front page had said.
"Have they gone mad?"
"The two princesses were finally caught this morning after a drunken spree that lasted most of last night."
Celestia looked over from the print to the picture of Luna and Cadence sleeping on top of each other in the destroyed remains of a coffee shop. With a small chuckle, she let out a quiet, "Idiots" under her breath.
Author's Note
This is just a dumb little idea that got fleshed out into this mess, but hey, it's a sign that I haven't died off yet! Sure everypony is out of character, but I just felt like getting this out there. 
Edit: it seems that many people would like this to be fleshed out a bit more. I may try to when I get some free time and have thought through how I want it to go. And thank you for all the support!