The reason I'm writing this is that during the early BETA, fishing was actually useful i.e. provided ingredients for provisioning and alchemy. Or so I read on the internet, where there's still fishing guides based on BETA information.

Let's be honest, I know it's a fantasy world, but i can't imagine 10 different cultures, almost all their lands bordering the ocean, uniformly having decided not to eat fish.

Even in it's current useless state, I enjoy fishing immensely and have to keep myself from wasting too much time on it. Now here's my take on how to improve the experience/usefulness:

Reintroduce fish as provisioning/alchemy mats. Maybe fish oil could be a byproduct of cooking fish, then be used as a reagent, if it's a separate oil per fish species, or as a solvent if it's always the same oil.

Rare fish should be lootable again, not just a check sign in the achievements page. The drop/catch rate should not be increased, but the fish should be stackable (The greens can be caught fairly often, the blues are the really rare ones).

Include Rare fish in the highest level foods and their oil in the highest level potions and/or give them real value again, something along the lines of 150G per green, 500G per blue.

If they stay pure trophy items and do not receive a high vendor value, don't make them lootable until we have player housing, many walls for fishing trophies included.

This may not be high on all too many priority lists and of course, bug fixes should come first, but as soon as zenimax's staff (who have done admirable work so far, imho) have time to focus on future content, I'd really love to see a system similar to the one described.

TL;DR: Fish should be in provisioning and alchemy recipes.