Well, a major theme is the challenges they face. Since they are young, dealing with pregnancy brings up issues like financial concerns, finishing school, and their own personal growth. Also, there's the theme of acceptance. They have to accept the new situation and their new roles. For instance, Taiga might have to accept becoming a mother at a young age, and Ryuuji has to step up as a father - to - be. There's also the theme of how society views their situation, which some fanfictions explore in depth.
There are quite a few fanfictions out there about Taiga and Ryuuji in a pregnancy scenario. Some are really well - written. You can start by looking on popular fanfiction websites like Archive of Our Own. Just search for 'Toradora Taiga and Ryuuji pregnant' and you'll get a list of relevant stories. Some focus on how they deal with the unexpected pregnancy, their relationship growth during this time, and how their friends react.
One possible plot could be that Ryuuji wakes up one day as a girl. He, now she, has to deal with the reactions of her friends like Taiga. Taiga might initially be shocked but then grow more protective. Ryuuji would also have to adjust to new social dynamics at school, like how the boys interact with her now and how she fits in with the girls. She might struggle with wearing different clothes and having different expectations placed on her.
Perhaps an old acquaintance of Ryuuji's who has some unresolved issues with him. When Ryuuji turns into a girl, this person sees it as an opportunity to get revenge in a more devious way. They might try to manipulate the situation and the people around Ryuuji to make her life miserable, all while hiding their true motives from others.
Maybe not many people are interested in creating fanfic about them. Or it could be that their storylines don't inspire much imagination for fanfiction writers.
Overall, the Toradora manga is excellent. It has a compelling plot, great character development, and beautiful art. It manages to capture the emotions and relationships in a very realistic way.
Show their small daily interactions. For example, how they share the household chores or how they have a routine in the morning like having coffee together. These little things make the relationship seem real.