There is a theory according to which Im-sama is based on the Ancient Egyptian God Medjed, which appearance does look like the silhouette we’ve seen of Im without the crown.
In Ancient Egyptian religion, Medjed is a god mentioned in the Book of the Dead. His ghost-like portrayal in illustrations on the Greenfield papyrus earned him popularity in modern Japanese culture, including as a character in video games and anime.
E.A. Wallis Budge translated a passage of the Book of the Dead the Medjed that said:
“I know the being Mātchet [Medjed] who is among them in the House of Osiris, shooting rays of light from [his] eye, but who himself is unseen. He goeth round about heaven robed in the flame of his mouth, commanding Hāpi, but remaining himself unseen.”
The passage says he is “unseen” just like the way only a few people know about the existence of Im-sama in One Piece. Also mentioned Hāpi who was the god of the annual flooding of the Nile in ancient Egyptian religion and can be a reference to the sea and Im being somehow in control of it.
Medjed gained popularity after the British Museum held an exhibit on ancient Egypt at the Mori Arts Center Gallery in Tokyo in 2012, a few years before Im’s first appearance in the manga. Medjed became a sensation on Japanese social media due to the resemblance to a stereotypical ghost costume. The god was embraced by Japanese popular culture. He became an internet meme, as well as a character in video games and in anime.