First, you may already know my idea for a potential KFP 5 villain... he isn't a sea creature neither an insect but, in any case, I think it's a cool idea: in case you don't know about that, my idea is that of an Asian black bear warlord ( voiced by Benedict Wong from Raya) with a mechanical arm, so the second villain with artificial body parts after Shen, with a falcon sidekick ( I still have to find a voice for him) and a still unknown goal but, maybe, related to the Emperor of China ( a white tiger voiced by Dave Wong and Tigress' biological father, maybe with another, younger daughter, the Princess of China and Tigress' sister, with white fur like her father - born after Tigress left her family and was maybe presumed to be dead by the Emperor and Empress); his combat style would be a mix of physical strenght, his mechanical arm and, well, some supernatural abilities involving chi.
And, about the sixth villain, an interesting theory came to my mind: not only the second woman villain after the Chameleon but also the first young villain, about the same age of Po and possibly Zhen: I must still find a voice and a species for her but I think her fighting style could be also based on chi ( obviously different from Kai).