I would like to see one of a kind (one per world) legendary fish in the game. Fish that take multiple attempts to weaken and may even need to be found multiple times in the ocean. These Legendary fish could have interesting effects when placed in captivity. I Love collecting things in this game. Perhaps also a fish notebook with recorded fish captured and size?
Some examples:
Golden Koi: When placed mobs will gather around entranced by its beauty.
Rainbow Jelly: Changes colors. Has a radius of infinite light.
Zebra-spider-fish: Once a day, passively shoots out silk.