Not sure if I am the only one, but I feel compared to other professions that fishing is the slowest one of all of them to level. As the levels get higher and higher, the exp gain doesn't change at all as any fish caught still gives only 2xp.
My suggestion would be to allow the Winter stream event in the icerun area in the campaign to also award some fishing exp. You get 12 fish in this event and to balance things out, I feel like it should give you 6 fishing exp as a reward. This way people can grind up to lv20 fishing normally and then choose to grind the following levels through campaign or by normal means or both!