Well, it could have been one of Roger Rabbit's many enemies. Maybe it was someone seeking revenge or trying to gain some sort of advantage. The possibilities are endless.
It could be a variety of characters. Maybe it was an enemy seeking revenge or someone with a hidden agenda. But it really depends on the specific fanfiction plot.
Well - developed characters. A great fanfic would really dig into the personalities of Roger and Jessica. It should show their relationship in a new light, perhaps exploring their backstories more deeply or showing how they interact in different situations than in the original movie.
You can start by looking at fanfiction websites like Archive of Our Own. There are often a lot of fan-created stories about Roger Rabbit and Jessica Rabbit there. Just search for their names in the search bar, and you'll be presented with various fanfics to choose from.