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.
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.
Often, there is the theme of inspiration. Deku is very inspiring in the original story, and in fanfiction, he can inspire Shoto to be a better hero. Also, self - discovery. They both are on a journey to discover more about themselves, their powers, and their place in the hero world. In fanfiction, this journey can be elaborated on in different and interesting ways.
One common theme is redemption. Dabi, with his troubled past, often has a journey towards redemption, and Shoto can play a part in that. Another theme is family. Given their complex family relationships in the original story, fanfics often explore how they deal with family issues together.
No, it is not appropriate. Creating or distributing sexual fanfiction involving underage or fictional characters in a sexual context is unethical and often violates the terms of use of various platforms. Such content can also be considered a form of inappropriate objectification.