It could be that there's a doppelganger situation. The character in London has a look - alike in 90210, and the events that made people think it was impossible are actually related to this look - alike. Or, there could be some sort of virtual reality or dream sequence where the perception of the character's location is all messed up.
Well, perhaps there's some sort of time - space anomaly. Maybe a magical portal opened up that allows the character who is supposed to be in London to suddenly appear in 90210. Another possibility could be that it was all a big misunderstanding and the person in London was misidentified, and the real character is actually in 90210 all along.
The plot twist might involve a secret organization with the power to teleport people. They accidentally sent the character who should be in London to 90210. Or, it could be that the character has a hidden power or ability that they themselves are not aware of, like the power to be in two places at once, which is why it seems impossible that they are in 90210 when they were thought to be in London.
Well, it could be that the character who is supposed to be in London has a doppelganger in 90210. And the events that seem impossible are actually caused by the actions of this doppelganger. Maybe the main character gets some misinformation that makes them think it's impossible for someone to be in London when in fact, there is a perfectly logical explanation within the convoluted world of fanfic.
Another way would be to introduce some magical or science - fictional element. For example, the character in London has the ability to teleport or has a device that allows him to be in two places at once. This would add an interesting twist to the fanfic and also make the 'impossible' situation possible within the fictional universe.
Maybe he was using some sort of advanced holographic technology. So, it only seemed like he was in London, but in fact, he was in 90210 all the time. This would resolve the contradiction quite neatly.
Perhaps it's a business - related fanfic. People from different companies come to London for a meeting. There could be power plays, secret alliances being formed, and unexpected revelations during this London meeting. For example, a young entrepreneur might meet a big - shot investor at the meeting and try to convince them to invest in their startup.
Well, it could be about a character who enters a mysterious world where every riddle has an unexpected and strange solution. For example, a riddle about a door might lead to a portal to another dimension instead of just opening a regular door.
One possible plot twist could be that Sasuke, after seeing Sakura kiss someone else in the fanfic, realizes his true feelings for her. Maybe he had been suppressing his emotions all along, but this event makes him confront them. It could lead to a journey of self - discovery for him, where he has to decide whether to fight for Sakura or let her go.
A simple plot could be that Harry goes to London for a Quidditch tournament. While there, he gets lost in the city and stumbles upon a hidden wizarding library that contains ancient spells and prophecies. He then has to figure out how to get back to the tournament while also dealing with the knowledge he has gained from the library.
One possible plot could be that characters from two different fandoms accidentally meet during a big event in London. For example, a wizard from a fantasy fandom and a detective from a crime-solving fandom cross paths while investigating a mysterious occurrence in the city. They initially distrust each other but as they realize they are both after the same goal, they team up.
One possible plot twist could be that Naruto unlocks the Rinnegan in a moment of extreme emotional turmoil. For example, when he witnesses the death of a very close friend like Sasuke. This could trigger a dormant power within him that was previously unknown. Another aspect could be that the way he uses the Rinnegan is completely different from how others have. He might use it to bring peace in a more unique way, like using its power to erase negative emotions from people's hearts.
One idea could be that Naruto accidentally breaks into Ino's family's secret garden in a fanfic. This place might hold some important family secrets or a rare herb that Naruto needs for a mission. And Ino catches him there, leading to a whole series of misunderstandings and adventures.