This is a idea I'd like to see made into a story.

At a pony convention everyone gets something with a jewel/Crystal (necklace, bracelet) later in the main hall with the stage someone (maybe even Twilight Sparkle) puts magic into all the crystals and when worn it turns the wearer into a pony/ other mlp creatures and they turn back when taken off.

The crystal also bonds with the wearer so no one else can use it, it also changes shape to some how resemble the transformed wearer (like a cutie mark).

The transformed effect becomes becomes permanent if the crystal is worn to long, but as a safety feature the crystal can turn the wearer back into a human when worn but when taken back off they will turn back into a pony/mlp creature.

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Okay could work, what would the the plot of the story and what would the human do with it in their live? At some point the novelty would wear off for most of them, and will just stop using them, unless you are very creative about. And not to mention that some or many don't like the pony they become. I would guess that they could have the option of visiting area, but that would basically be playing tourists, and I don't think the more popular ponies would take kindly to be ask to do their repeat their signature catch phrases. I could imagine a number wanting to immigrate in Equestria for various reason. There is also the consideration that not all people using them are actually bronies and could use it to do illegal stuff thanks to the varying tribes power or being able to use human tech in Equestria, or try to escape the authorities in Equestrian . And that not to mention government using them to try to spy and influence Equestria or to try to unwrap the secrets of magic and artifacts; and there wouldn't be anything for Equestria wanting to counter it with doing the same. There is also the consideration that Twilight might have anterior motives in offering that to people, either good or bad for the wearers that can benefit her, but insists on making it look like it's a choice for them or that the choice is critical for what she is hoping for. Those are the various treat that I can come up with at the top of my head. Hope this help in developing your idea further.

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And not to mention that some or many don't like the pony they become.

I was thinking that the transformation is based on the person, Like I may turn into my OC because he is a version of me.

If you're talking about going to and from Equestria I wasn't thinking about that happening.

There is also the consideration that Twilight might have anterior motives in offering that to people

Twilight is just giving people specifically Bronies something they'd like.

Some of the other things you said are outside the scope of my idea.

I like the idea and thinking what the various reactions could be. You should try it and see what the community thinks. In going to follow you so I get notified when you post it if you do. Sounds interesting!