Me, designing it a long time ago: Having an optional boss fight as a random chance to trigger when you fish in a certain spot with a certain type of bait is fun! Cool idea!
Me, trying to playtest everything again: oh god just let me see the boss, my bag is so full of fish!
I just successfully tested having events check if specific actors are in a particular party slot and then changing image to match. This will let me make scenes take into account who is in the party at the time and arrange them accordingly - this is important since I'll have extra actors beyond current party. The script used is the same from MV and is compatible with my current core plugin setup in MZ. Thank goodness.