1. Master Angler is nice but only applies to the base game, so now you're looking at hundreds of other fish you catch in DLC areas for no reason other than getting the angler achievement for that particular DLC area. The new fish have some really interesting names and potential attributes which could certainly expand the interest in those DLCs when integrated with particular events.

2. I suggest adding another title, such as Grand Master Angler, which would cover catching fish in many other DLC areas up to Greymoor or something. Achieving the Grand Master Angler achievement could, for example, allow a ship (versus a boat) as a reward, and to be docked at a Notable Home or wherever a person could place it. As the DLCs move on, Grand Master Angler II, GMA III, and so on.

3. Also, we have green and blue fish...but no purple and even golden fish.

4. Create some fish (such as blue fish catches) to be animated furnishings so we can place them in homes where we have waterfalls and pools.

5. And like another person suggested about an aquarium, but I'll take it a bit further than that....look into a furnishing type that is strictly water, available in different sizes, and the user simply uses available furnishing materials (stone slabs, wood, trees, etc.) to create their own aquarium and put their animated fish into. So, instead of a fixed aquarium, the user is left to design however large a space they want to fill with their collected fish. The devs can make it binding so only fish could be put into the water for easier effects and less rewrite times for potential contamination of other objects and errors in a home.

6. Explore looking into a Fishing Event.

I would like to reiterate that ESO is more than just killing things...it's also an "achievement experience" for many of us who want to sometimes take a break from PVP and PVE fighting and just relax at a nearby area and fish.