I personally hope it will be used as ingredient for crafting. Finally a way to create our own energy potions. Or just completely remove potions from the game.
Also I hope we get a fishing catalog where we can complete a list. Some fish only come in certain area's or even in certain times of the day / time of the year.
Oh Bug catching. Please reffer to the genius game Animal Crossing. <3
Some funny quests can be done with fishing as well. Think off: fishing competitions and lure the bears that terrorize the village out of theor cave with a pile of fish. (Group event maybe) the more fish collected the stronger / more bears come out. Better drops. (Dynamic level scaling.)