One common theme could be adventure. They might embark on some kind of journey together in the fanfiction. For example, exploring a new land or solving a mystery.
I'm not sure specifically where to find bridget x king gristle fanfiction. You could try searching on some popular fanfiction websites like Archive of Our Own (AO3). They have a large collection of various fanfictions, and you might be lucky to find some related to this pairing there.
One moral could be the danger of excessive greed. In many troll stories, the king troll often wants more and more - more power, more treasure. But this insatiable greed usually leads to his downfall, teaching us that being too greedy can bring negative consequences.
The moral could also be about the significance of respecting others. If the troll king disrespects those around him, whether they are his subjects or other creatures in the story, it can lead to unrest and his own downfall. Respecting others creates a positive environment and can prevent conflict, as shown in the story where the king's lack of respect may be a contributing factor to his negative fate.
They might get really excited and start comparing it to their own experiences in the Trolls world. Maybe they'll laugh at the funny parts or feel inspired by the creative interpretations in the fanfiction.
Well, in fantasy fiction, cave trolls differ from other trolls in several ways. For one, their physical features might be more suited to cave living. They could have a thicker hide to protect against cave - related hazards. In terms of behavior, cave trolls are usually more territorial about their caves. While other trolls might be more nomadic or have a wider range of movement. Also, the diet of a cave troll is likely to be more limited to what is available in or near the cave, such as bats or underground fungi, as opposed to other trolls that might have a more diverse diet depending on their habitat.
I'm not sure specifically who Bridget Barton is within the context of Fantastic Fiction. It could be an author, a character, or someone related to the Fantastic Fiction community. More information would be needed to give a more definite answer.