Well, obviously Batman and Nightwing are the main characters. Batman, the dark and brooding protector of Gotham, with his vast array of gadgets and skills. And Nightwing, the acrobatic and more light - hearted hero who was once Robin.
The theme of family also runs strong. The Bat - family is complex, and when Nightwing and Batman meet, it can highlight the family aspect. They might have disagreements like any family members, but ultimately they are there for each other. This is seen in fanfictions where they protect each other from threats both external and internal to the family.
Another possible plot is that they discover a long - lost secret about Gotham's past together. This could lead them on a journey through the city's underbelly and old, forgotten places, uncovering clues and facing challenges along the way.
I'm not sure who the absolute best is, but some popular ones might be those who have a deep understanding of the Batman and Nightwing characters. Maybe authors who have been following the comics for a long time.
One interesting plot could be that they team up to take on a new and powerful villain in Gotham. Maybe this villain has some unique powers that force Batman and Nightwing to combine their skills in unexpected ways.
A really good fanfiction could be centered around a time when Nightwing doubts his place in Batman's life. Batman then has to prove to Nightwing that he is like a son to him. This could involve Batman opening up about his own fears and insecurities, which is not something he usually does. It would make for a very interesting read as it shows a different side of their relationship.
A fanfiction might explore the idea of Batman and Nightwing going on a 'father - son' road trip. They could be driving through Gotham and its outskirts, having deep conversations about their past, their future, and what it means to be a hero. Along the way, they face some small - scale villains and use their combined skills to defeat them, all while strengthening their father - son bond.
One great 'Nightwing meets Batman' fanfiction is 'The Reunion' on FanFiction.net. It has a really interesting plot where Nightwing and Batman have to team up to take down a new and dangerous criminal in Gotham. The author does a wonderful job of capturing their dynamic relationship.
It can symbolize protection. Batman is the mentor, and when he carries Nightwing, it shows his instinct to keep Nightwing safe from harm, whether it's physical harm from villains or other threats.
One possible fanfic could involve a battle against a powerful villain. Nightwing might get injured while trying to protect Gotham. Batman then has to step in not only to defeat the villain but also to take care of Nightwing. He could use his advanced medical knowledge and resources in the Batcave to nurse Nightwing back to health.