Eyeless Jack can be developed by showing his softer side when interacting with the baby girl. For example, he might be more patient and gentle, which is a contrast to his usual menacing reputation. This can add depth to his character.
In this fanfiction, Marshmallow could be a significant character. He's a large and somewhat intimidating snow creature created by Elsa. But with the baby on the way, he could become a gentle protector. He could stand guard outside their home to keep any potential threats away. Also, the trolls from the Frozen universe could play a role. They are wise and could offer their knowledge on how to take care of a baby with magical powers. They might even perform a special ceremony for the unborn baby.
They can develop their relationship through their shared care for the baby. For example, when the baby is sick, Elsa uses her powers to keep it cool while Jack tries to cheer it up. This mutual effort in taking care of the baby can bring them closer.
Jack was a man who was rather sly and liked to play tricks. He made a deal with the Devil and through his wits, he managed to avoid being claimed by the Devil immediately. But his actions led to him being barred from both Heaven and Hell.
Jack's personality would likely become more innocent and naive. As a baby pooka, he might be more curious about the world around him without the usual wariness he has as his normal self. He could be more easily frightened by things that he wouldn't normally be afraid of as an older Jack. Also, he might be more impulsive in his actions, just going for things that catch his eye without thinking much about the consequences.
One common theme is family. It shows how Jack and Elsa form a family with their baby and all the love and care that comes with it. Another theme can be the combination of their powers in the baby. Their baby may have a unique blend of Jack's wintery powers and Elsa's ice powers.
Perhaps in this form, Jack Frost is befriended by a young human child. The child doesn't realize he's Jack Frost and just thinks he's a really cool and strange little animal. They go on adventures together in the winter wonderland, with Jack Frost using his powers in small, unnoticeable ways at first.
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.
I'm not entirely sure as the statement is rather unclear. It could potentially be a fanfiction where Jack Kline, a character perhaps, is depicted as a baby in some way within the fictional narrative.