I've been fishing my whole life, and I noticed that most fish in this game are (relatively) small. I remember visiting a town that had a fishing museum, and in the front window there was a board with fish skulls that were over a foot long. The fish themselves were supposed to be over 2 metres long. They weren't any special types either, they were just huge versions of regular fish like cod and yellow-belly. They also cant be found any more, since they can only get to those sizes in areas that are almost never fished in.
I thought that similar giant fish could be added in H&H, as a super rare object in fishing. Either there is an incredibly small chance of catching them when doing regular fishing, or a new type of fishing could be added purely for them, but they would still be really rare. Also, they would either take up a large inventory space (2x4) or are like large animal carcasses and must be butchered in a similar way. Their meat would have very high FEPs and they could also give other items like a giant fin or fish scales, which could be curios or gilding. Basically it was just an idea to give fishing more of a purpose. Also add an experience event.