Seems like a perfect time to implement this with One Tamriel, as the level barriers for travel between zones will be lifted, so the quest giver could send us all over Tamriel in search of rare fishies.

Some of the quests could even be combat related, like sending us to the Black Marsh to fish up and kill a rumored swamp monster that has been (allegedly) terrorizing the locals. Bring me the head of the Swamp Thing! Or send us fishing with M'aiq the Liar as they say fishermen are the biggest liars.

The rewards could be things like new special fish-related provisioning recipes. Or maybe some special rare bait that actually gives an increased chance at a green or blue fishie. Or some fishing motifs allowing players to craft fishing gear.

Also, how about a slot for fishing poles?
This could be a great cosmetic enhancement. And ZoS could even put some cosmetic poles on the Crown Store, so a chance for revenue.

The fishing in the game is good; it's a fun and relaxing (sometimes frustrating when going for the achievements) way to explore the world. But it could be great.