In the fanfiction, Jack and Arcee could be magically merged into one being for a short time, and during that time, they create a new form of life. They then have to deal with the consequences of this unexpected creation and learn to be parents to this new being.
One idea could be that Jack discovers a hidden power within Arcee that she was not aware of herself. They then have to go on a quest to find out more about this power and how it can be used to help the Autobots in their fight against the Decepticons. Another plot could be that Jack gets lost in a dangerous part of Cybertron and Arcee has to brave all sorts of perils to find and rescue him.
One idea could be that Baby Bumblebee gets lost during a battle and Arcee is the one who finds him. She then has to protect him while they try to find their way back to the Autobot base. Another could be a story where Arcee teaches Baby Bumblebee some important combat skills in a fun and unique way. Maybe she creates a sort of training course just for him.
One common theme could be friendship. Jack and Arcee might form a strong bond in these stories. Another could be growth, as Jack might grow and learn from his experiences with Arcee.
One key element is the relationship between Arcee and Jack. It's often about how they interact, like how Arcee protects Jack while Jack offers her a different view of the world. Another element is the action. Since they are in the Transformers world, there are usually battles against Decepticons. For example, in many fanfictions, Jack might be in a dangerous situation and Arcee has to rescue him.
There could be a variety of 'jack and baby arcee fanfiction' out there. Some might focus on their adventures together, while others could explore a more emotional bond between them. You really have to dig around different fanfiction sources to discover the ones that appeal to you.
One idea could be that their baby grows up with a love for the sea, just like their parents' fateful encounter on the Titanic. Maybe the baby discovers a hidden diary that details their parents' story and becomes inspired to preserve the memory of the Titanic in a new way, like through art or historical research.
Jack, a young and brave human, and Arcee, the intelligent and powerful Cybertronian, are the central figures. Since it's about a baby, they will probably have to interact with other characters like the other Autobots who may offer help or advice on taking care of the baby.
Well, it's possible that there could be some out there. However, this is a rather unusual concept. It might not be a common theme in the general Arcee and Jack related fanfiction. You'd have to dig deep into the various fanfiction communities and use specific search terms to find out.
One possible plot is that an experiment gone wrong results in Arcee and Jack having a baby. This baby could have special abilities, being part human and part Transformer. The fanfic could then focus on how they deal with the reactions of both the human and Transformer communities. Jack would have to teach the baby about human values, and Arcee would teach about the Transformer way of life.
Another idea is that the baby girl is a new creation in Eyeless Jack's realm. She has unique powers that are slowly emerging. As she grows, Eyeless Jack takes on the role of a mentor, guiding her to control her powers and use them for good, or at least for their own survival in that strange and often cruel world.