This is thematically similar to an idea I brainstormed with a bunch of college friends ages ago.
Instead of an RPG with a fishing minigame, a full fledged fishing game with a semi-hidden JRPG side mode. Like one of those weirdly detailed Dreamcast fishing games, then halfway through you start fishing up vaguely Lovecraftian things. Things get weirder until someone dies in an accident during a tournament, and when that lake reopens you fish his undead body out and start some JRPG shit with Cthulhu and Atlantis. Complete that and it goes back to fishing game like nothing happened.
This was before the trend of "secretly a horror game" indie games, and the tone was more goofy than it sounds.
Anyway, I'll have to check this out!