I haven't been around too long but after doing some searches I didn't see any fishing focused suggestions, so I hope I'm not duplicating anything here.
Overall access to fish: Fish should be like dirt, imo. If you're in a river or lake there should be fish there... no more vacant lakes, especially. This is not proposing to eliminate the current "jumping fish" spots, but simply add fish to the areas without jumping fish. Like dirt, the default would be q10. It would be nice (but not necessary) to have a random chance (5%?) to catch a "big one" of higher quality, softcapped by gear as usual.
Fishing spear: I would like to see fishing made more accessible to newbies for an early food source. So, I propose the fishing spear... simple to make from sticks (and maybe a bone or rock.) This would be a primitive fishing instrument that only functions while standing in shallow water with a reduced chance of a fish "bite" compared to a fishing pole, or some other method of making it clearly inferior past the beginner phase.
School (aka, jumping fish) functionality: The current fish hotspots should remain as the primary access to higher quality fish as they already are today. Also the chance of a fish "bite" should be much higher in a school than in plain water. However, I would like to see a little more randomness in quality; at least in the form of a small chance for a "big one" of a higher quality even if the chance is very small.
Fish fight: This applies only to pole fishing, not spears. Once you get a "bite" an interface similar to the ranged accuracy meter will appear with yellow at the bottom, green in the middle, and red at the top. The fish will "fight" by moving the bar up and down, maybe it moves more crazily for higher quality fish (or more "valuable" kinds of fish?) If you click in the green you have a good chance to land the fish. Too much into the yellow and you have a chance to lose the fish, too much into the red and you might lose your line, hook, or lure. It might take a few clicks to finally land a fish. Several elements of the fight could vary depending on your gear, skills, and the type and quality of the fish that's on the line. I haven't thought all of that through. The moral of the story is that fishing is no longer an afk activity, and because of that....
LP: It's a common complaint that fishing LPs really suck. With the removal of AFK fishing I propose making it more competitive. I think I'll just leave it at that.
Fish types: Right now what you catch is largely a mystery in this game. I think in some ways that's a cool feature so everyone can have their fishing secrets. However, I don't really like the lack of randomness in some situations. There shouldn't be any situations, highly IMO, where you catch one kind of fish 100% of the time. Also I think gear quality should not be a factor in the type of fish caught. Time of day, phase of moon, body of water type, and lure/bait should continue to be main factors of fish type caught.
Fancy gear: Things like gold hooks and lures should be amazing, but yet not block good fish from being caught by lesser gear. Maybe just something like sturgeon, eel, and salmon bites go way up with expensive lures? Maybe that already happens, but I don't think it's fully recognized by the community so I'm mentioning it here. Or maybe the fish fight is easier with the fancier gear?
More fish recipes: Put simply, more fish recipes. Yeah, everyone wants more recipes for everything, right? Heh. So I'll just throw in some ideas to finish up my list. Dried fish - put fish on a drying rack and get a little different FEPs compared to roasted fish, that alone adds a lot more choices. Unspecific fish recipes - Just like fish sticks, recipes that can use any kind of fish. There probably shouldn't be too many of these, but maybe one or two more would be nice.
Anyway, that's just some things I thought of while staring at my inventory while in a boat. =)