One popular theme is the adventure aspect. Fans often write about the baby growing up in a world full of time - traveling and alien encounters, just like the Doctor and Rose had. Another theme is family. It could be about how the Doctor, Rose, and their baby form a unique family unit, dealing with the challenges of being part of different species. Also, the theme of protecting the baby from various threats, both from within their own universe and from other dimensions, is quite common in these fanfictions.
A great idea for this fanfiction could be that Rose starts having dreams about their future child. These dreams could give them clues about the baby's purpose or the challenges they will face. The Doctor would be very interested in these dreams and try to analyze them scientifically.
In the fanfiction, Rose's pregnancy could be a catalyst for the Doctor to finally settle down in one place for a while. Maybe they find a peaceful planet where they can build a home and prepare for the arrival of the baby. Their friends from their adventures might come to visit and offer their support. It could also lead to some interesting cultural clashes as they combine their different upbringings and knowledge to raise the child.
Writers often start with the baby as a symbol of hope. As the story progresses, the baby might start to show curiosity, much like the Doctor. For example, reaching out to strange objects or showing an interest in the TARDIS. Then, as the baby grows, it could develop a sense of justice, similar to how Rose always stood up for what was right. Maybe the baby starts to question things around it, just as the Doctor is always full of questions.
Adventure is another theme. Since they travel through time and space together, many fanfics have them going on all sorts of exciting adventures, whether it's saving a planet or exploring a new era. It adds to the overall charm of their relationship as they face these challenges together.
The Doctor would likely be both excited and terrified. He'd be extremely protective. He might start modifying the TARDIS to be a safer place for Rose and the baby, adding things like special shields or a nursery area.
Another interesting one is 'Parallel Worlds'. In this fanfiction, it explores different parallel universes where the Tenth Doctor and Rose's relationship takes on various forms. It's full of creative ideas and really makes you think about the possibilities of their relationship.
One popular one could be 'A New Beginning' where they start their married life on a new planet, facing all kinds of adventures together. It's full of their typical banter and sweet moments.
Love is a very common theme. Their love for each other is what binds them as they face the new challenges of having a baby. It's shown in how they support each other through the difficult times.
The kiss in fanfiction can be a catalyst for change in their relationship. It can bring out different sides of their personalities. For example, the Tenth Doctor might become more vulnerable around Rose after the kiss, showing his softer emotions. Rose, on the other hand, could become more confident in her relationship with the Doctor. This new - found confidence can lead to her taking more initiative in their adventures together, as well as in their relationship.