Gon in fanfiction can be very single - minded in his pursuit of goals, just like in the original. He might be a bit more naive at times, which can lead to interesting plot twists. Killua is often portrayed as having a complex emotional side. He struggles with his past as an assassin and his loyalty to Gon. His cool exterior may crack in some fanfictions, showing his true feelings.
Another situation could be when a new male character enters the story. Let's say a powerful and handsome demon arrives. If Kagome shows any interest in this new character, like asking about his powers or his world, Inuyasha would feel jealous. He would puff out his chest and try to show that he is more powerful and a better choice for Kagome. He might even start a fight with the new character just to prove his point.
One place to find fanfiction about Blake getting jealous could be Archive of Our Own (AO3). There are often many different takes on this theme. For example, in some stories, Blake might get jealous when another character gets too close to her love interest. Another example could be in a story where she is jealous of a new friend who seems to be taking up all of her partner's time.
One possible scenario could be that in the fanfic, the main character says 'I love you' to someone else, perhaps a new friend or acquaintance. Yamato, who has feelings for the main character, then becomes jealous. Yamato might start acting distant at first, not wanting to show their vulnerability. But as the story progresses, they can't hold it in anymore and confront the main character about their feelings, leading to a big emotional moment in the fanfic.
A typical situation is when Bulma has to attend a social event with an old friend who is a wealthy and charming man. Vegeta, who is not used to such social settings and is insecure in his own way, would see this man as a threat. He would watch them closely at the event, fuming silently and getting jealous because he thinks Bulma might be attracted to this more 'civilized' man.
It adds an interesting layer of drama. Readers like to see characters they know in new situations, and Sonic being jealous is a new side of him. It can create conflict, which is essential in any good story.
In a fanfiction where Tigress gets jealous, it could be that there's a new student at the Jade Palace. This student seems to get all of Shifu's attention. Tigress, who has always been Shifu's star pupil in a way, feels left out and jealous. So she starts to train even harder, but also gets snippy with the new student. Eventually, she realizes that her jealousy is unfounded when she sees that the new student just wants to learn and doesn't mean to steal Shifu's attention.
Yes, there are likely some fanfictions out there with such a concept. Fanfiction writers often come up with creative crossovers. You can try searching on popular fanfiction websites like Archive of Our Own or Fanfiction.net.