One possible plotline could be the two of them trying to keep their relationship hidden from Ron, Ginny's brother. Since Ron is Harry's best friend, they might be worried about how he'd react. Another could be them sneaking around Hogwarts to have private moments together, like meeting in the Room of Requirement.
There could be a plotline where Harry and Ginny's secret dating actually helps Harry in his fight against Voldemort. Their love gives him an extra boost of strength and motivation, but they have to be careful not to let Voldemort or his Death Eaters find out, because they might use Ginny as a way to get to Harry.
In some fanfictions, the secrecy of their relationship might be threatened by a nosy student or a jealous rival. Maybe a Slytherin student suspects something and tries to expose them. This could lead to all sorts of tense situations as Harry and Ginny scramble to keep their secret safe.
One plotline could be that they secretly married to protect Ginny from a dark wizard who has a vendetta against Harry. By being secretly married, they think they can keep Ginny out of harm's way. Another could be that they did it on a whim during a romantic getaway and then decide to keep it a secret just for a while to enjoy their private time together.
Hermione would be a great addition. She could be the one who accidentally discovers their secret and then has to decide whether to keep it or tell Ron. Neville could also be involved. Maybe he stumbles upon them in the greenhouse while they're having a secret moment and he becomes their accomplice in keeping the secret.
One interesting plotline could be how they deal with their different house loyalties. Ginny is a Gryffindor and Draco is a Slytherin. Their relationship might be a secret at first, and they have to hide it from their friends and families who have strong prejudices against the other house. For example, Ginny might struggle with telling Harry and Ron about her new relationship, as they have had many conflicts with Draco in the past.
Some fanfictions might have a plotline where Draco and Hermione are secretly dating and they communicate through enchanted objects. Since they can't be seen together often, they use magic to send messages or small gifts to each other. This adds an element of mystery and secrecy to their relationship. Another idea could be that they meet in a hidden part of Hogwarts, like an old abandoned classroom or a secret passageway. Here, they can be themselves and share their feelings without the fear of being discovered. There could also be a plotline where they help each other overcome personal issues. For example, Draco helps Hermione deal with the pressure of being the best in her class, and Hermione helps Draco break free from his family's expectations.
One possible plotline could be Ginny being influenced by a dark magical object that makes her turn against Harry. This could lead to a series of conflicts as Harry tries to figure out what's wrong with her and how to save her. Another plotline might involve Harry and Ginny being caught in a dark prophecy that forces them to make difficult choices that seem against their nature. For example, they might have to choose between saving each other or the wizarding world.
One interesting plotline could be the paparazzi misinterpreting a friendly moment between Harry and Ginny as something more romantic, which then causes chaos in their wizarding world. For example, they might take a picture of Harry helping Ginny up after she falls during Quidditch practice and spin it into a love affair story.
One interesting plotline could be how Harry and Ginny handle their kids' magical education. For example, they might have different ideas on whether to send them to Hogwarts early or let them study at home for a while first. Another could be the family's adventures during holidays, like exploring new magical places together.
One interesting plotline could be their post - war life. How they deal with the trauma of the war and build a normal life together. For example, they might struggle with nightmares but find comfort in each other.
One interesting plotline could be Harry and Ginny having a secret date in Hogsmeade. They might discover a hidden alley with a magical shop that holds a key to solving a mystery at Hogwarts. Another could be that they encounter a strange creature near Hogsmeade and have to work together to keep it from causing chaos in the village. And also, perhaps they meet an old wizard in Hogsmeade who tells them a prophecy about their future together.
Some fanfictions have a plotline where they form a secret club or group. They might be working against a new threat that the official Hogwarts authorities are not aware of. It shows their loyalty to each other and their ability to take matters into their own hands to protect the wizarding world.