There was no clear connection between Sanguine Red's novels," Tengen " and " Tengen 1 ", which could form an independent series. However, judging from the name, Tengen could be considered a follow-up to Tengen 1 because Tengen was usually used to refer to the first work in the series. However, this was not absolute, because the name connection did not necessarily mean that there was a direct causality between the works.
One common theme could be adventure. Since Inosuke is a rather wild and adventurous character, fanfics might involve him and AOI going on some sort of exciting journey together, perhaps exploring a new and dangerous area.
Some other platforms like Wattpad might also have such fanfics. However, the quality can vary. You could also check out some anime - related forums. Sometimes fans share their own written fanfics or recommend ones they've read. Keep in mind to respect the rules of each platform when looking for these fanfics.