So, by now we can all agree that Sea of Thieves will most likely get a fishing mechanic. The devs have acknowledged that it's one of the most requested things, and we could use something to do while we sail from one place to the next.
Unfortunately, I've personally never been a fan of fishing mechanics in games. They're normally either very boring "press button when rod bends, you get fish", or the tedious "fish is on the line, spam these QTE to reel it in without it getting off". Both of which I find gets boring after a while. To keep fishing interesting, this is how I would like the mechanic to work:
The game introduces 2 new items, a fishing rod and a harpoon. To fish, you cast a line into the water, and hit a button to hook the fish. Once it's hooked, you can either hold the button to reel the fish in more, or put the rod into a holder on the boat (Basically just a circle to keep the rod there.) While it's in the holder, the fish can swim from side to side, and towards the boat, but not back out to see. You then have to throw the harpoon at the fish to try and hit it. If you do, you caught the fish. If not, hold the same button for fishing to bring it back, and repeat until you catch the fish, or after X time the line breaks.
This I feel would add a unique and interactive aspect for fishing. Plus, if the words Drunken Sea of Thieves Spear Fishing doesn't sound fun, I don't know what to tell you.