One great storyline could be them going on a secret mission together. Maybe they have to infiltrate an enemy base where their speed and archery skills are both crucial. Another could be a story where they get stranded in an alternate dimension and have to rely on each other to get back home. It could explore their friendship and how they grow to trust each other more deeply.
To write their character development, consider their backstories. Artemis has her own family issues and Kid Flash has his insecurities about being a hero. Have them open up to each other about these things. For example, in a dangerous situation, Artemis might share a bit about her family with Kid Flash, making him understand her better. And Kid Flash could show his vulnerability to Artemis, which would make her see him in a new light. This way, their relationship and individual characters can grow and develop over the course of the fanfiction.
One story that comes to mind is 'Midnight Encounter' which beautifully depicts the moment between Nightwing and Artemis. The author does a great job of building up the tension before the kiss. It's on AO3 and has received a lot of positive feedback from readers.
One plot idea could be that they are undercover on a mission as a couple. As they get into character, they end up having a real moment and kiss. This kiss then makes them question their true feelings for each other.
One great story idea could be that Raven senses a dark and ancient evil awakening. She tries to stop it on her own but realizes she needs help. So she reaches out to Kid Flash. At first, Kid Flash is a bit skeptical because Raven's powers are so different from his. But as they start to investigate, they find out that the evil has the ability to nullify Kid Flash's speed in certain areas. Raven then uses her powers to protect Kid Flash and together they find a way to defeat the evil by combining their unique abilities.
Well, when writing this type of fanfiction, it's important to stay true to the essence of the characters. For Nightwing, his leadership skills and sense of justice should shine through. For Kid Flash, his impulsive yet good - hearted nature should be there. You can add some original characters or twists to the existing DC lore, but make sure it doesn't go against what the fans already know. Also, use descriptive language to bring their superpowers to life. For example, when Kid Flash runs, describe the blur he leaves behind and the wind that follows him. And finally, give the story an emotional arc. Let the readers feel something, whether it's excitement, sadness, or joy.
A common theme is the exploration of their different powers and how they complement each other. Nightwing's acrobatic skills and combat expertise combined with Kid Flash's super - speed can lead to some really exciting scenarios in fanfiction. Also, there can be themes related to their personal growth. They could face challenges that force them to develop as characters, either separately or together.