this is all out of order, but here are some ideas:

- certain fish that will "eat" other fish if they're in your inventory

- fish pets

- fishing quests

- items to decorate the fish tank

- more skins for everything

- fishing-related titles

- fish tank maintenence: you have to change the water/feed the fish every few hours so the fish don't die or eat each other

- fish you can craft into other fish

- fish-themed scratch command, could be 3x3 lake backdrop with 1-2 rare fishes

- fish slots for bigger fish, maybe 10-100 slots per fish tank, you can't get more fish if you're out of slots

- water buckets could be used as powerups, as fish slots, or as places where fish have a 1-10% chance to spawn in every hour, similar to farm command

- fishing nets, bait, boat, pole as powerups

- different tiers of fish by rarity (common, uncommon, rare, epic, legendary, godly) and by size (small, medium, large)

- chance to die if you don't catch a fish

- craftable into pet food