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.
Most BSD light novels often have complex characters. Since they are based on literary figures, they bring in the depth and complexity of those original personas. For example, a character might have the same traits as a famous writer but with a unique twist for the BSD universe.
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.
The appeal of all BSD novels lies in their ability to create immersive worlds. They draw readers in with their unique concepts and engaging storylines. For instance, the way they handle character relationships is very captivating. The conflicts and reconciliations between characters make for a great read.
Honestly, there's no definite answer on when the BSD manga will end. It all comes down to various factors like the author's inspiration, the popularity of the series, and the publisher's decisions.