Fishing seems to be there just to kill time. Just like those Sightseeing logs, you get minion and 2 achievements but thats it.
Any sort of competetion would be kinda silly, since Big Fishing is all RNG. Sometimes u have fish that requires 50 less Gathering than what you have and you get 20x Fish gets away... message and then u catch one HQ where u barely had enough Gathering. Its weird.

I'd say if you want to change something about them, make all big fishes have chance for some sort of materia (from Desynthesis).

Now when looking at the Desynth info, they give mostly Fine sand which is like worth 1 gil.
They went to better direction this patch, stuff giving materials for i110 crafted armors and Crit/Dete/Parry/SS IV materias.

I'm just fishing because of Master Caster title, and might as well get the ironworks tool while im at it.