It could be a new dark wizard that they see in the future. Maybe someone who was overlooked during the fight against Voldemort but has been secretly gathering power.
One possible plot could be that they see a future where the wizarding world is in great danger again. So they start to train young wizards secretly to prepare for the upcoming threat.
Well, perhaps they will discover that magic has merged with Muggle technology in the future. Harry and Ginny could be amazed to find flying cars that are powered by both magic and electricity. They may also encounter new forms of dark magic that they have to fight against, or they might see a future where the wizarding world has achieved complete peace and harmony, and they play an important part in maintaining that peace.
Well, one possibility could be that the kids from the future come back to prevent a great disaster in the present. Maybe a dark wizard that was defeated in their time has somehow found a way to be reborn in the past (the present in the story), and they need to stop it before it's too late.
One possible plot could be that the child from the future comes back to prevent a great disaster. Maybe a dark wizard in their future time has found a way to rewrite history and the child has to make sure Harry and Ginny meet and fall in love at the right time. The child might also have some special powers inherited from their parents which they use to face various challenges in the past.
Another theme could be the passing on of values. Harry and Ginny would want to instill in their kids the values they hold dear, like courage and loyalty. This can be shown through various interactions and situations in the fanfiction.
Obviously, Harry himself is a central character. Hermione and Ron are also very likely to be in it as his closest friends. Dumbledore may also play an important role, as he often guides Harry in the original stories. He could be the one Harry confides in about his new ability to see the future.
One option could be time - travelers from a different future. Maybe there are some wizards from a parallel future who want to change the past to suit their own needs. They might be opposed to Harry and Ginny's kids because they are trying to protect the original timeline. These antagonists could use some really complex time - magic to try and outwit the kids.
It could be a new dark wizard who has risen to power in the future and is trying to change the past to gain more power. This villain might be jealous of Harry's fame and Ginny's influence in the wizarding world, and by changing the past, they hope to rewrite history in their favor.
It's possible that when they meet their kids, they realize that their family has a special destiny in the wizarding world. Their kids could be the key to uniting different magical communities or solving an ancient mystery. This could lead to a series of adventures where Harry and Ginny, along with their kids, embark on a journey to fulfill this destiny.