Wrote the beginning of this story to help flesh out the idea, see if I wanted to continue it. Might come back and finish it after Apple Hospitality, but I also might move on to some other ideas I have :shrug:
 
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>The blinding light and ringing in your ears slowly fade.
>You're on all fours and try to stand up, but your head feels woozy.
>You settle for sitting on your legs.
>As your vision clears, you realize you're in a small library with unusually curvy shelves.
>"Well, that was unexpected."
>That voice.
>...No, it can't be.
>You whip your head around to look, but your vision blurs again and a sudden wave of dizziness causes you to fall on your side.
>"I thought it only showed me the other dimension, not pulled stuff through."
>You hear the sound of pages rustling... and a faint shimmering sound.
>Another voice speaks, painfully familiar, "But what IS it?"
>"I don't know. Looks like some kind of weird, hairless monkey."
>Your vision is finally clearing up again.
>You carefully rise to your knees again and look slowly around at the source of the voices.
>...It really is her.
>Purple fur.
>Navy and magenta mane.
>A horn.
>It's fucking Twilight Sparkle.
>She looks... somehow different though.
>Either it's too subtle or your head is too messed up for you to figure out exactly how. 
>You cough.
"T... Twilight?"
>Her head jerks up from the book and she stares at you.
"Twilight... Sparkle."
>She slowly turns to face you.
>"How...?"
>You turn your head and see the small dragon always by her side.
"Spike..."
>He backs away from you, "Uh... this is creepy."
>She whips back around to her book.
>"I agree, this was a mistake. Give me a second, I'll send him back."
>Send me back?
>Back to Earth?
>Back to your shithead father?
>Back to your asshole 'friends?'
>Back to the world plagued by corruption, stupidity, and boredom?
"W-Wait..."
>Dammit, it's so hard to speak.
>Twilight glances at you, grimaces, then turns back to her book.
"Twilight please..."
>Her ears flatten.
"You can't send me back."
>"Twilight, this thing is really creeping me out."
>"Me too, Spike, but I can't find the return spell."
>You take a deep breath.
>This might be your only chance.
"Twilight, please listen to me, if only for a moment."
>If you really focus, you can talk almost normally.
>...It is then you realize you are naked.
>You look down, startled, and frantically cover your genitals with your hands.
"Oh jesus. Twilight, I'm sorry. Can I cover up with a blanket or something?"
>She looks up and sighs, then her horn glows and a towel floats over to you.
>Actually floats, glowing light purple, just like the show.
>Dumbfounded, you just stare at it in awe.
>"Ahem?"
"Oh, sorry."
>You hastily grab it out of the air and wrap it around your hips.
"Ok, can I talk?"
>She turns back to her book and says, "You have until I find the return spell."
>Shit.
"Okay, look... I don't know how, but somehow back on my planet, there is a... a story of this place, of Equestria. So far it seems to be exactly correct... I mean, I haven't seen much so far but correct me if I'm wrong: you are Twilight Sparkle, daughter of Twilight Velvet and Night Light, brother to Shining Armor, faithful student to Princess Celestia, and this is Spike, your baby dragon assistant whose egg you hatched as your initiation into Celestia's school for the magically gifted. Celestia sent you to live here, in the Golden Oak Library in Ponyville to learn about friendship. 
>While you spoke, Twilight slowly turned to face you, forgetting her search in her wonder.
"...Back on Earth, my home, I thought it was just a story, not real. I had no reason to believe it was real."
>You take a deep, shuddering breath.
"Even so, I loved it. I love everything about this place, Twilight. I love the ponies, I love the land, I love the adventures, I... I wished for so long that it was real and that I could visit."
>You gesture around you.
"But it IS real... and I'm here. The place I come from, Earth... I hate it there. I tried to convince myself for a long time that it was a better place and I was just being cynical, but really, I can't stand it. I hate almost everything about Earth."
>You look down at your hands in your lap and try not to choke on your words.
"...and you want to send me back?"
>You look up at her again, feeling tears start in your eyes.
"Twilight, please. I can't go back. It was hard enough thinking Equestria was just a story. If I had to live on Earth and know that Equestria was real, I don't think I could survive. I think I would go mad. I think... I don't think I could live with myself."
>Her tail flicks and her ears flatten again, but she doesn't react otherwise.
>The tears are flowing openly now.
"I *can't* go back. I just can't, Twilight. It would kill me. Please. I'll do anything. I'll go anywhere, work anywhere, help anyone - er, anypony do anything they need. Whatever it takes to stay, I'll do it. Anything! Just please don't send me back, I'm begging you!"
>Her expression doesn't change. Obviously she isn't impressed and still only thinks you're creepy. She'll send you back any second now, and your life will be over.
>Your arms grip your chest as the sobs wrack your body, tears falling to the floor.
"Please... don't send me back... Twilight, Please..."
>You mumble over and over again.
>A minute goes by, your voice growing weaker with each repetition, eventually stopping altogether.
>The only sound now is your pained, quiet sobs.
>Finally, Twilight quietly says, "Spike, take a letter."
>You hear a faint rustle.
>"Dear Princess Celestia. I apologize for such a demand, but I have an urgent matter which requires your immediate attention. Please come to my library in Ponyville as soon as you are able. Your faithful student, Twilight."
>You hear a whoosh of flame and mere seconds later, a warm glow envelops your right side. 
>Despite your condition, you can't help but straighten and look a its source.
>The sun shines brightly through the window and seems to be intensifying.
>Suddenly, the light seems to solidify into the shape of a pony.
>No, the shape of a mare.
>Princess Celestia.
>Right in front of you.
>She's the size of a real horse back home and her horn is as long as your entire arm.
>Her mane and tail billows in non-existent wind and like Twilight, she seems to look just slightly different from what you expect.
>Her eyes widen when she notices you and she turns to Twilight.
>"This is what you've called me for?"
>"Yes. I was testing the portal view spell and accidentally pulled this creature through."
>"Twilight, this hardly seems to be a problem. Simply send it back and learn to control the spell better."
>You break into fresh sobs.
>They pause for a few seconds before Twilight continues, "...That's just it, Princess. The creature, whatever it is, speaks our language and just spent several minutes begging me to let it to stay here in Equestria. It claims it was miserable back on... I think it said "Herth," it's home. Somehow they have a story of Equestria, accurate down to even recent events about me."
>Celestia sounds surprised, "Now that is interesting."
>"He says they're just stories, thought not to be real, but he had already fallen in love with our world, even knowing it was fictional. He claims he couldn't bear to live in his Herth that he hates so much now that he knows Equestria is real, that it would drive him mad."
>You hear hoofsteps and two slender white forelegs come into view as you are bent over your knees.
>Your head begins tingling and you feel an odd pressure on the underside of your chin.
>With astonishment, you realize Celestia was using her own magic to lift up on your chin.
>You allow your head to tilt up and see she had lowered her head to your height, her expression soft.
>Her voice tender, she murmurs, "Can you speak?"
>You try to speak but only let out an odd croak. 
>You clear your throat, swallow, and sniff, then eventually mutter.
"Yes."
>"What is your name?"
"...Anonymous. Anon for short."
>"And what do we call you? Your species, I mean."
"I'm a human."
>"Alright, Anon the human. I cannot promise you can stay here yet, but we will not send you back just yet. You must come with me and we will talk about this story of Equestria."
>A faint hope blossoms in your chest and Celestia steps back and looks at Twilight.
>"I will return shortly after I question this human for a bit."
>Twilight nods seriously, "I await eagerly. I'm very curious about this... human."
>The same warmth radiates from the window, but this time it seems to curl around you and envelop your other side as well, the side facing away from the window.
>The light intensifies so rapidly you are forced to close your eyes.
>Just as rapidly, the light and heat fade, and you open you eyes again.
>And gasp.
>You are in the Castle of the Two Sisters, kneeling on the ground in your little towel, in front of the very throne of Equestria. 
>There's no mistaking the long hallway with its stained glass windows.
>Celestia stands beside you, but lays down next to you immediately.
>Her face gentle, she asks quietly, "Now, Anon, tell me. How much do you know about Equestria?"
>Suddenly you have an ugly thought.
>Twilight's still in the library and doesn't have wings.
>Somehow, you're in the *middle* of the series.
>That means things you do and say could potentially change the outcome of future seasons.
"I... I mean no disrespect, but I'm not sure it would be safe to speak openly about such things."
>"You can hardly be less open than murmuring to me and me alone in the center of Canterlot, the safest place in Equestria."
"And uh... is Canterlot safe from time paradoxes?"
>This gives her pause, "How do you mean?"
>You take a deep breath.
"How long ago did you send Twilight to Ponyville?"
>"Just a few days ago, why?"
"The story back on Earth... it shows the future. The future of Equestria, I mean. You sending Twilight to Ponyville is literally the beginning of the story."
>She looks down thoughtfully, "That is extraordinarily interesting. It's true that time is not something to tamper with lightly, but tell me, were there any humans in this story of Equestria?"
"No."
>"Then we may well already be on a timeline different from what you know. Further changing the timeline should not be anything to worry about."
"But... how can you be sure?"
>"I can't." She sighs, "But... such is the weight of a crown. My choices are send you home and effectively sentence you to madness and death, leaving the timeline the same and hoping it is a happy story you tell on Earth, or keep you here and try to use your knowledge to help... yet possibly change the future enough that such knowledge is rendered useless."
>She looks away for a moment, obviously lost in thought. 
>You wait awkwardly, trying to clean the tears with Twilight's towel without exposing yourself.
>Finally, the Princess turns her head back to you.
>"My decision is to risk your knowledge, but only in short bursts. Tell me, what is the next significant event?"
"Nightmare Moon returns."
>She nods solemnly, "I saw the signs, that is why I sent Twilight to Ponyville. She must make friends to weild the Elements of Harmony."
"Yes."
>Her billowing tail seems to flicker rapidly. You suppose that's her equivalent of flicking her tail.
>"Does she succeed?"
>You remember that, short of sending Twilight to Ponyville, Celestia does effectively nothing, so her knowing shouldn't be an issue.
"Yes. You don't help her at all, though. She has a lot of trouble making friends, but eventually, her and five other ponies find the Elements of Harmony just after Nightmare Moon returns and Twilight just barely figures out the sixth element in time to stop her."
>Celestia nods, "And what happens to Nightmare Moon?"
"She is returned to her normal self, Princess Luna, your sister, and at your behest, returns to rule with you in harmony."
>She looks up, closes her eyes, and takes a long, slow breath.
>You see one small tear leak from her eye.
>Suddenly, she stands up before you, her wings unfurl, and she looks down at you from what seems a great height, speaking in a stern tone.
>"This is your test, Anon. You have told me what you deem the future. As you have spoken, I will not act, even as the signs of Nightmare Moon draw near. I am trusting you in this. If your prediction proves accurate, you may consider yourself a permanent resident. Until that time, you will remain here in the castle, secret and out of sight. No one but myself and my most trusted guard will know of your existence; I will remove your existence from Twilight and Spike's memory, hopefully to minimize any impact of your existence on this timeline. If you are wrong, you will be returned to your home dimension and what follows will be only your fault.
>You stand before the Princess, forced to look up slightly into her eyes.
"I understand. If the time fluctuations lead to problems and I am wrong... I can only apologize."
>You bow.
>She nods solemnly and the warm light envelops you again.
"Thank you for this chance. I can never express how grateful I am."