Here is a theory about why Oda chose "Snake"man as Luffy's new Gear 4th form.
Katakuri used an attack called "Kagami Mochi" in chapter 883. Kagami mochi, which literally means “mirror mochi rice cake,” is a traditional Japanese new year’s decoration.
It is called "mirror mochi" since the shape is similar to bronze mirror which was considered as a treasure by ancient Japanese. They believed that by double decking such noble items, their fortune will also double. It is also said by some folklore researchers that the shape of Mirror Mochi is based on sacred snake coils. Maybe Luffy as "Snake"man fighting "Mochi" guy in "Mirror" World is not coincidence in Oda's mind..