One common theme is probably the 'opposites attract' scenario. For example, a very bubbly and outgoing character falling for the more reserved Suga. Another theme could be 'finding love in unexpected places', like on a tour or during a random encounter.
There are fanfics where the 'lov capture deku' scenario leads to an exploration of Deku's character growth. For example, while captured, he might start to understand the motives of the LOV members on a deeper level, which in turn changes his approach to heroism in the story. This can add a lot of depth to his character.
If Luke were kidnapped, in the context of fanfiction, it might be the work of a new, unknown group. This group could be after the knowledge Luke has about the Force. His kidnapping could be a way to force him to reveal secrets. The Jedi Order would be in chaos, and his students, if he has any at that point, would be left without their master. They might try to find him on their own, leading to their own character development and adventures.
Well, you can check out Archive of Our Own (AO3). There are many fanfics there about BTS Suga in a loving situation. Just search for 'bts suga in lov' on the site and you'll find a variety of stories written by different fans.
You can start by searching on popular fanfic websites like Archive of Our Own. Just type in 'lov capture deku fanfic' in the search bar and it will show you relevant stories.
It could be a new villain who has a soft spot for Bubbles. Maybe he's a misunderstood bad guy who just wants some attention. He might see Bubbles as a symbol of purity and kindness that he lacks in his life, so he kidnaps her in a rather desperate attempt to be close to her.
In such a fanfic, perhaps Bubbles would initially be terrified when kidnapped. But as time goes by, she might discover some kindness in her captor. Their relationship could develop gradually. For example, the captor might show her a different side of the world she never knew. And Bubbles, being the sweet and innocent one among the Powerpuff Girls, could start to have feelings for this person who is different from the usual villains she faces. This would create a complex dynamic where she has to balance her new - found feelings with her loyalty to her sisters and the city they protect.