One common theme could be mystery. Since 'light gets caught' might imply something unexpected or strange happening with light, it could lead to a mystery - like what caused the light to be caught and what are the consequences. Another theme might be transformation. The event of the light getting caught could potentially change a character or a situation in some way.
I haven't come across much 'light gets caught' fanfiction myself. But you might want to check out relevant fan communities on social media platforms. They often share and discuss interesting fanfics in this area. Maybe someone there can point you to some great ones.
In some fanfictions, Dumbledore is portrayed as having a lot of secrets. If he gets caught, it could be related to his past. Maybe some old enemies find out where he is and catch him. Then he would have to deal with his past coming back to haunt him. His friends might try to rescue him, but it would be a very difficult situation for him, and it could change the whole dynamic of the fanfiction story.
One possibility is that Dumbledore gets caught spying on a group of Death Eaters. This would put him in a very dangerous situation as the Death Eaters are extremely cruel and loyal to Voldemort. He might be tortured for information about the Order of the Phoenix. However, Dumbledore is very resourceful and would probably be able to hold his own against them for a while, giving his allies time to figure out a rescue plan.
Since I haven't actually read this particular fanfiction, it's hard to say for certain. However, based on the title, we can assume that Richie is the main focus. The Deadlights are likely some sort of supernatural or dangerous element. It could involve Richie being trapped in a world created by the Deadlights, facing his worst nightmares perhaps, and the story might be about his journey to break free from this terrifying situation, whether through his own strength or with the help of others.
The story would take a different turn if Dumbledore gets caught. His capture could be the starting point of a new adventure for other characters. They might set out to prove his innocence or find out the real reasons behind his actions. This could involve a lot of detective - like work, exploring different parts of the wizarding world, and uncovering secrets that were long buried. It could also lead to a change in the relationships between the characters, as they might question their loyalty to Dumbledore or to those who caught him.
There's also a chance that Dumbledore gets caught by a group of rogue magical creatures. For instance, if he was exploring an area that was supposed to be off - limits to wizards and the creatures there saw him as an intruder. They might not be as rational as wizards and could hold him captive without really understanding who he is.
Even some of the Hogwarts professors or students could catch Dumbledore. Maybe a curious student stumbles upon Dumbledore in a restricted area where he's doing something strange. Or a professor who has a different view on how to run the school catches Dumbledore in the middle of an action that they think is wrong for the school's environment.
In some cases, when Dexter gets caught, the people who catch him might try to rehabilitate him. They could think that there's still some good in him and attempt to change his ways. However, Dexter might resist this change as his nature is deeply ingrained. This could lead to a power struggle between him and those who want to reform him, creating a lot of drama and tension in the fanfiction story.