Great idea, I'll write that down for the sequel. For this I think I am going to make the flame bigger depending on the burp size :)
then the electric eel tail with a shark should give her a mega mouth like a gulper eel where she devours the screen. So I'm glad you are doing a sequel. Hopefully you manage to put some deep sea moments in with a black screen and much more glowing fish and glowing sharks maybe an angler fish mode or a black dragon fish mode or a comb jelly... Actually a comb jelly may be hard to animate... Im not sure how difficult it would be for you to make a boss shark for her to eat as well? That maybe calls smaller sharks she eats to get bigger until she can eat the boss shark.
I also think she should be able to sing a song to hypnotize the sharks as a standard mermaid and even in the sequel if it goes that for she could grow legs and feed on beach Gowers turtles coconut crabs snakes etc then she could find a surf board and she could surf and eat anything in her way. While she is on land she would use the song to hypnotize people, perhaps she could go to Komodo island and feed on the dragons If she gets knocked off she swims for a while still able to eat prey before her tail grows back.
This is how I think of it in terms of old sega m drive games I used to play.
If I was going to go further I would unlock a prehistoric mode with mosasaur lioplurodons etc she could eat and power up into. By the end there should be selectable versions like you have
Prehistoric
Surf
Deep-sea
Island legs
Komodo
Gulper
Flame
Eel
Maybe a jelly mode
You could have secret chars like a medusa that freezes anything looking at her a Cerberus with three upper bodies one tail. A hydra with multiple tails that grow mouths on so she can't be eaten from behind, a kraken with tentacles and a jet also cannot be bum rushed. A harpy that dives like a pelican but can glide also. And an orca that grows super fast when she eats sharks. These are my ideas for now. I hope you keep working on this game thanks for making it