One idea could be that they are adopted by a powerful but reclusive wizard. This wizard has his own motives for taking them in, like using their potential for a grand magical plan. Another plot could be that they are adopted into different families at first and then reunite later, realizing they have a special bond that goes beyond blood relations. Maybe they discover that their adoption was part of a prophecy protection mechanism, to keep them safe from the dark forces that are after them.
Perhaps Harry is adopted by a Muggle family who are aware of the wizarding world. They could be a family of scientists who study magic from a scientific perspective. Harry would grow up with a unique understanding of magic, seeing it both as a wonder of the wizarding world and something that can be analyzed. This might make his time at Hogwarts very interesting as he tries to blend these two views of magic.
A possible plot is that they are in bed and start arguing about a minor thing like who left their wand on the wrong side of the bedside table. But then it turns into a playful banter and then a passionate moment as they realize how silly the argument was. Another idea is that they are in bed getting ready to go to sleep and they start talking about their families. Hermione tells Ron about her Muggle parents and how she misses them sometimes, and Ron shares stories about his large family at the Burrow. This conversation deepens their bond as they fall asleep in each other's arms.
A great plot could be that they discover they are twins later in their Hogwarts years. This revelation could cause a stir in the wizarding world. They might then start to explore their family history together, finding out secrets about their parents that were previously unknown. Maybe they find out that their parents had some special mission that they now need to continue as twins.
Well, a plot could involve Hermione being influenced by a powerful dark magic item. This item warps her mind and makes her see Harry as a threat to the wizarding world. So, she betrays him by revealing his secrets to his enemies. Or perhaps she gets jealous of Harry's popularity and power and that leads to her betrayal in a moment of weakness.
One plot idea could be a magical adventure where the four of them go on a secret quest together. Maybe they discover an ancient magical artifact that requires all their skills to unlock its secrets. Another could be a time - travel plot where they end up in a different era of the wizarding world and have to adapt and work together to find their way back.
A really interesting plot could be that right before the wedding, a dark wizard from Ron's past tries to disrupt it. Hermione and Ron, along with their friends, have to fight off the threat while still making sure the wedding goes on as planned. It would add a lot of action and adventure to the fanfiction.
Perhaps there's a curse that has been placed on the Great Hall. It makes the food turn into strange and sometimes dangerous things. Harry, Hermione, and Ron take it upon themselves to break the curse. They start by researching in the library, but soon realize that they need to explore the Great Hall more closely. They discover that the source of the curse is an old grudge held by a former house - elf who used to work in the hall. They then have to find a way to make amends and break the curse.
In the fanfiction, after the kiss in the tent, they could face some internal struggles. Ron might be worried about not being good enough for Hermione, who is so brilliant. Hermione, on the other hand, could be concerned about how their relationship will affect the group dynamic. They then have to work through these issues, with some sweet and awkward moments along the way as they grow closer.
A really fun plot could be them going on an adventure during their engagement. They discover an old, forgotten magic related to love and engagement, and they have to solve some puzzles to understand it better. This adventure could strengthen their bond and also give them some unique wedding ideas.
Another plot could be that Naruto is adopted by an older ninja who has retired from active duty. This ninja has a lot of wisdom and secrets. Under his guidance, Naruto discovers a new power within himself that was hidden all along.