In some fanfictions, Harry and Ginny might fight side by side, using their respective magical skills. Ginny could be shown as a fierce fighter, using her Bat - Bogey Hex to great effect against the enemies, while Harry uses his Patronus to protect them from Dementors if any are present during the battle.
There could also be fanfictions where they are in charge of different parts of the battle strategy. Harry, with his more overall knowledge of Voldemort's plans, might be leading the defense against the main force of the dark side. Ginny, on the other hand, could be in charge of protecting the younger and more vulnerable wizards and witches. She would use her strength and courage to keep them safe from harm, all while waiting for the moment when she can rejoin Harry in the main fight.
One possible scenario could be that after the intense battle, Harry and Ginny find a quiet corner in the ruins. They look at each other, all the emotions of the battle still fresh in their eyes. Ginny reaches out and touches Harry's face gently, and then they slowly lean in for a kiss, a kiss full of relief that they are both alive and still have each other.
They might get married and start a family right away. In fanfiction, it's common to see them settling down and having kids, living a peaceful life after all the chaos of the battle.
One possible scenario could be that Ginny meets an old acquaintance who has changed a great deal and she starts to have feelings for this person while still being with Harry. Maybe they share a common interest that she feels Harry doesn't understand. However, this goes against Ginny's strong character in the original series, so it's a very different take in fanfiction.
In some versions, it could be that Harry has to die to break a dark curse placed on Ginny. He discovers that his life is the key to saving her and willingly gives it up. He might have a premonition about Ginny's fate and decides that his death is the only way to change it. He then confronts the source of the curse and allows it to take his life instead of Ginny's.
In some fanfictions, they might go on adventures together. Exploring the wizarding world more fully, perhaps seeking out new magical creatures or ancient magic. They have a thirst for discovery after the chaos of the war. Harry's knowledge and Ginny's courage would make them a great team on these excursions.
One possible scenario could be Ginny trying to control who Harry spends time with. For example, she might not like him being around Hermione so much because she's jealous. Another could be her trying to control Harry's actions in the wizarding world, like which quests or missions he undertakes. Maybe she thinks some are too dangerous and tries to stop him forcefully.
It's also possible that they meet at a Weasley family gathering that Harry attends. Ginny, being the youngest Weasley, is a bit shy at first but then Harry says something funny or kind, and that starts their interaction.
One possible scenario could be that Harry finally finds the courage during a quiet moment at Hogwarts, perhaps in the Gryffindor common room by the fireplace. He might stutter a bit at first, but then clearly say his feelings. Another could be that it happens during a dangerous adventure when he realizes he can't hold his feelings in any longer, fearing he might lose Ginny. Maybe he confesses after Ginny has been in some sort of danger and he realizes how much she means to him.
One possible scenario could be that they meet in the ruins of Hogwarts. The war has just ended, and there's a sense of relief and newfound hope. Harry spots Ginny among the crowd of survivors. Their eyes meet, and without a word, they rush into each other's arms and share a passionate kiss. It's a moment of pure emotion, a celebration of their survival and their love that endured through the dark times of the war.
Well, perhaps Ginny is trying to make Harry jealous by wearing his clothes in a fanfiction. She might put on his old Gryffindor scarf and walk around the common room, catching Harry's eye. Or, in a more adventurous scenario, Ginny has to disguise herself as Harry for some reason and has to wear his entire set of clothes, which leads to all sorts of comical and dangerous situations as she tries to act like him.