Because these are such an awesome ideas it deserves a consolidated post of its own.
Introduce variable fish sizes.
This is pretty self-explanatory same fish just in different sizes. Maybe you can get an extra filet or get less depending on the size. You could also add a unit of measurement of the size.
Deep Sea Fishes
Big Fish like Tuna, Grouper, Swordfishes, Sharks, maybe dolphins too
Smaller fish like Herring, Sardines, Squid, Skates
Introduce a customizable rod and reel
Using different materials to make different rods and reels. For example a basic rod and reel will allow you to catch the basic fishes, to catch larger fishes you would need to upgrade to maybe a bigger pole with a steel reel with a woven silk line. Want better luck at fishing maybe a rose gold and steel reel on a fancy rod. Who’s to say if you actually have any better luck with it or not but at least rose gold would have a use in something.
Mountable Fish.
Because everyone wants to mount that huge sturgeon they caught on the wall so they can brag about it while their getting raided.
Bait Fish Traps.
Used for catching new bait. Most fish love to eat minnows. Shoot you could even add In little schools of minnows so we know where to place the trap.
Crab and Crawfish Traps
Add in some traps to catch various crustaceans a large trap for catching crab and lobsters in the shallow ocean and a smaller version for use in lakes and rivers to catch crawfish.
Sand Fleas.
This is a good bait crustaceans that lives in the sand on the beach. It can be found randomly while digging sand at the beach.
A Fish Basket.
Can be worn on your back or placed at the prow of a rowboat giving you a little extra storage for the days catch ( Can only contain fish )
Return Fishing Song
As suggested by turtlehermit