One common theme is friendship. Since Rangiku has a lot of friends in the 'Bleach' universe, fanfictions often explore her deep bonds with them, like with Ichigo or Renji. For example, stories might show how she helps them through tough times.
One common theme is the exploration of their relationship within the Soul Society. It could be about how they support each other in their duties as Soul Reapers.
Her past and how it has shaped her is also a frequent theme. Her time with Gin and how that relationship has influenced her actions and decisions in the present. Additionally, her place in the 10th Division of the Soul Society and how she interacts with the other members of the division. This can include everything from training sessions to just casual conversations around the division headquarters.
One common theme is their past relationship. Writers often explore how they met and the early days of their friendship or something more. For example, stories might focus on Gin's true feelings when he first saw Rangiku.
I've also heard of a fanfiction that focuses on a more comical side of a Naruto - Rangiku Matsumoto encounter. For example, Naruto is confused by the strange new world he's in and Rangiku has to explain things to him in a very humorous way. Their different personalities, Naruto's naivete and Rangiku's laid - back yet strong - willed nature, make for a lot of comical situations.
A common trope is the 'fish out of water' scenario. Naruto being in the Bleach universe and trying to figure out how things work while Rangiku helps or hinders him. Another is the power - exchange trope where they teach each other their unique abilities. For example, Naruto shows Rangiku how to use shadow clones in a creative way, and she shows him how to use her sword's spiritual energy.
There's also 'Rangiku's Adventure Beyond the Seireitei'. This fanfiction takes her on an exciting journey outside the known areas, full of new challenges and character development. It really expands on her personality and abilities.
Another theme is the discovery of new powers. Given Naruto's ninja abilities and Rangiku's own skills in her universe, some fanfictions explore what happens when they combine their powers or learn new ones from each other. This can lead to some really exciting action scenes and unique plot developments.
One common theme is secrecy. Given their characters, their relationship often involves hidden motives and unspoken words.
Another theme is loyalty. There are stories that show how, despite their differences, both Gin and Matsumoto have a sense of loyalty to their own beliefs or to each other in certain situations. For example, Matsumoto might be loyal to her idea of justice while Gin has his own form of loyalty to his motives, and how these clash or align in the fanfiction.
Friendship is a common theme. Given their different personalities, their growing friendship can be explored. Another theme could be growth, both in terms of power and as individuals.