I have done a +18 level fishing pole from +10 level. According to my experiences i have some suggestions to make fishing system more attractive.
1) Its hard to catch Gold Crucian Carp and its bonus is really high. I suggest to catch it easier according to that decrease its bonus to %10 skil resistance.
2) Its hard to find minnows in game to catch rare fishes. So maybe we can catch rare fishes in a low posibility with worm.
3) There are useless fishes that gives mp points, moving speed and etc. Maybe there will be new fishes with new bonuses. I suggest skil damage, strong vs all character fishes in an acceptable ratio like %10 or less.