One of the main themes is probably time. Given the '55 minutes' in the title, it might explore the concept of limited time and what can be achieved within it. Another could be self - discovery, as characters often face situations that make them question who they are and what they are capable of.
One similar light novel could be 'Bungo Stray Dogs' series. It has an engaging plot with unique characters. Another might be 'Kekkai Sensen' which also has elements of the supernatural and interesting character dynamics. And 'Noragami' is also a great choice with its own take on the supernatural world and character relationships.
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.
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.