In 'Crimson Peak' baby fanfiction, a possible theme could be the cycle of life and death. Given the movie's macabre and spooky nature, a story about a baby could explore how life is fragile and yet resilient in the face of death that looms large in the 'Crimson Peak' universe. Another theme might be discovery. A baby in this world would be constantly discovering the strange and often terrifying things around them, much like the main characters in the movie did, and this could be an interesting theme to explore.
One possible theme could be family heritage. If the story focuses on the characters as babies or having babies, it might explore how the dark family secrets in 'Crimson Peak' would be passed down or affect the new generation. Another theme could be protection. Just as the main characters faced danger in the movie, a baby - centered fanfiction could show how the parents or guardians protect the baby from the same or new threats.