I'm not sure specifically as there could be many different interpretations. It might be a fanfic that focuses on characters in Harry Potter using certain means to achieve their ends, like a character using magic in an unexpected way to reach a goal.
There's no way to tell without reading the fanfic. However, it might feature some of the more popular secondary characters from the Harry Potter books like Luna Lovegood or Draco Malfoy. They could be given a more central role in this fanfic, with the story revolving around their own means and ends.
The main characters could include some of the new generation as well. For example, Albus Potter or Scorpius Malfoy. They could be at the center of this 'war' in the fanfic, perhaps dealing with the consequences of their parents' actions. Ginny Weasley could also be a main character, as she's a strong witch in her own right. And then there are the Hogwarts professors. Professor McGonagall could be leading the defense of Hogwarts in this war - like situation, and Professor Flitwick could be using his magical expertise to help the side he's on.
Well, a typical plot element might be the discovery of a new, powerful magic item or spell. In this 'This Means War' fanfic, different characters or groups could be fighting over it. It could be that the item has the potential to change the balance of power in the wizarding world. Additionally, betrayal is often a great plot element. A character who was thought to be on one side might switch, creating chaos and fueling the 'war'. And of course, love interests can also play a role. Two characters from opposing sides might fall in love, adding an extra layer of complexity to the conflict.
Terrified means to describe extreme panic and fear, causing people to feel extremely shocked and terrified. Calmness meant that one's thoughts or spirit were calm without uneasiness or suppressed feelings. One had suppressed or re-restrained one's feelings and calmed down.
One great 'Our Flag Means Death' fanfic is 'The Pirate's Heart'. It delves deeper into the relationship between Stede and Blackbeard. The author does an amazing job of capturing their personalities and the chemistry they have. It adds more backstory and emotional depth to their bond.
Definitely the two male leads. Their competitive and comical relationship is the heart of the movie and can be explored further in fanfic. You can show more of their backstory and how they became such rivals.
Xuan Lan referred to viewing all things in a profound and pure state of mind. It was an intuitive way of understanding. It originated from a method of understanding Tao proposed by Laozi. He believed that only by abandoning all distracting thoughts and prejudices and keeping his heart as clear as a mirror could he observe everything, understand Tao, and experience its essence. Xuan Lan's specific method was to eliminate the interference of perceptual experience, language concepts, and desires, allowing his heart to experience and visualize everything in peace. Xuan Lan was also used as an important term for literary thought, pointing out that literary creation and appreciation should have a special state of mind that transcended desire and utility.