Yes, actually 2 Mermaids are planned.
Viewing post in Amelia's Stranded Adventure comments
I think at some point I'll add something like a Normal and Casual difficulty. Right now I'm still looking for the sweet spot, and I wouldn't want to spend too much time working on such modes unless I know the game is in a good spot balance-wise. Otherwise I might spend too much time adjusting numbers instead of getting content ready.