1. Fish need to have more applications than they currently do. I think they're in a pretty good spot for alchemy, but I feel that they need to have more of a presence in higher level food than they currently do. This will create more demand for different fish and more people may take up fishing. Also, things like the fish gutting from FF11 that can reward gil or a useful item rarely would create more demand for certain types of fish that are current underutilized.

2. Baits in which fish favor and fish distribution by area need to be improved on. I compiled a spreadsheet a long time ago tracking which fish favor which bait, and for most areas all the same fish respond to the same baits. Even when fishing at the sweet spot for a certain fish with favored baits, you'd still get things you weren't targeting more often than not because they liked the bait too.
These are really my only issues with Fishing... I don't mind how it works since I think it's something that is fairly relaxing which isn't a bad thing. It definitely could be much faster-paced though. There should be a bit more depth with the bait and stuff like weather/time of day/moon phase as well as some things like Inside the Belly where you can work towards various rewards by hunting specific fish.