We should avoid focusing on the 'lemon' aspect as it may involve improper content. For regular Kancolle - Ushio fanfics, one characteristic could be the blending of different story elements. Kancolle has its own lore with shipgirls, and when Ushio is introduced, the fanfic might explore how his character meshes with this world. It could also play with different genres, such as adventure or mystery, while still staying true to the core identities of the characters from both the Kancolle and Ushio concepts.
It's fiction. Kensuke's Kingdom takes you into a fictional world crafted by the author's creativity. There's no real-life basis for the story or its elements.
One common theme is friendship. Ushio and Tora have a very special bond, and many fanfictions explore how this friendship grows and endures through different situations. Another theme is adventure. They often go on various quests in the original work, and fanfictions expand on these adventures.