4.X is introducing swimming and if HW's theme was sky then SB should be the sea expansion Yoshida mentioned before. As a result, I thought to mention a suggestion I've always had for fishing in this game - sport fishing.
This comes particularly from my time playing Archeage. If you haven't played it, it was a much more crafting/agricultural based game. It's a very different game and I don't expect a lot of the better parts of it to fit in FFXIV, but it's fishing system would be perfect here.
That is to say, the current system doesn't need to change since Archeage had what we have with added layers to it.
First of all, sport fishing.
This isn't a unique feature - it's even in FFXV, not to mention Yakuza or any other game with a fishing mini-game ever. Basically after you hook a fish, you would enter a mini game where you would match the fish's movement so that the line doesn't break. You loosen if it's too tense and reel while the fish rests.
I've always liked this simple game whenever I've seen it appear. I honestly expected extend it in FFXIV:ARR's launch because of how involved crafting was and even Minin and Botany is more engaging than the current fishing (not that I mind the relaxing fishing aspect as well).
Like I said, Archeage had something similar to our current system so it doesn't need to go anywhere BUT the context sport fishing appeared should be matched so it doesn't clash which leads to the next suggestion - Fishing Boats!
Sport fishing was only a thing in that other game when you went on a boat and fished in the open ocean. This is one of the most fun things I've done in a MMO ever. Getting a boat was an effort, like making an fc airship.
The big boats could store multiple big fish. This was needed because unlike regular fishing, these would not fit in your inventory. Instead you had to carry it on your back and you would be weighed down while having one. You could only have one at a time so having a big fishing boat was essential to make profitable voyages. And you needed a few people to help fill it up cause each catch took quite some time.
This fish would then be turned into a fish stand for gold. It was a lot per fish if you got the best ones.
It was such a wonderful social aspect and looking at Diadem and airship building, I can see the parts are all there for it to fit in perfectly in FFXIV.
This kind of thing would help fill out the gathering end-game (at least for fishing) and make it more social as well. I think we could use that.
SE doesn't need to follow it to the letter, they could make it their own.
But I'd really love a fishing mini-game (sport fishing) as well as being able to go on a boat on the open sea with friends to gather.