The unique world - building makes 'world trigger novel' interesting. It has a distinct setting with the concept of the Neighbors and the Trigger system.
The 'world trigger visual novel' probably delves deeper into the backstories of the characters in the World Trigger universe. For example, it might explore how the agents of Border came to be, their training, and their personal struggles. It could also provide more details about the Neighbors, their culture, and their motives for invading Earth. It offers fans a chance to experience the story from a different perspective, with different narrative choices and paths.
Osamu Mikumo is quite popular. His growth as a character in the original series makes him a great focus in fanfiction. He can be written in various scenarios where he overcomes more challenges. Yuuma Kuga is also popular as his unique abilities and his mysterious past offer a lot of room for exploration in fanfiction.
One interesting concept could be alternative character backstories. For example, a fanfiction might explore what led a side character to become an agent. Another concept could be different rankings systems within the Border organization. It could also be about new types of Triggers that are created by the characters in the fanfiction.
The unique world - building. It creates a world where there are different dimensions and threats from outside. The idea of the 'Triggers' which are the weapons and tools used by the characters is really cool.
There is also Yuma Kuga. He is a very strong and unique character with special abilities. His combat skills and his background play important roles in the story development. He is from outside the world as we know it in the context of the novel, which makes his character full of mystery and interest.