Putting some valuable or rare items in the stomach contents of the fish, or even just junk items worth some vendor gold, would be a little bit more of an incentive to keep fishing. Would also feel a bit rewarding at times, too. Kinda like in WoW. You'd be sitting there fishing, relaxing after a hard day's work IRL (note: GW2 fishing is far from relaxing), when you catch a normal fish but only it says "bloated" in front of the name. Getting a chest to open or gear or mats or vendor junk for gold. Ah, yes! Those were the relaxful fishing days....