One common theme is self - discovery. The original characters often find out more about themselves and their capabilities in the world of Arpeggio of Blue Steel. Another theme is friendship. They form bonds with the existing characters from the series.
Iona is a crucial character. She is a Fog ship who develops a special connection with Gunzou and starts to question her own programming and purpose. Another important character is Kongou, a powerful Fog battleship with her own personality and motives.
One idea could be exploring the backstories of the different shipgirls in more depth. For example, how they came to be and their experiences before joining the fleet.
There are quite a few out there. 'OC: A Different Path in Arpeggio' is a fanfiction that takes the OC on a path that diverges from the main story line in an exciting way. It shows how the OC's unique abilities and personality can change the course of events. Also, keep an eye out for fanfictions that are updated regularly as they tend to have better story development.
The strategic battles in the 'Arpeggio of Blue Steel Light Novel' are also quite engaging. The way the different ships with their own capabilities and weaknesses face off against each other, like how they use their stealth, firepower, and maneuverability. It's really exciting to read about their naval combat strategies.
I recommend a doujinshi derivative novel called "Comic: From the Armored Warrior to Not Being Human." It involves elements from many worlds such as the Legend of the Galaxy Hero and the Armored Warrior Nawa. Moreover, the main character has the ability to not be human. It looks a little similar to the arpeggios of the blue steel. I hope you like this fairy's recommendation. Muah ~😗