Imagine Harry being able to make plants sing. He could create a beautiful concert in the Hogwarts gardens with all the plants chiming in. Or he could use plants to create a floating garden in the sky above Hogwarts for a special event. Also, what if he uses his power to make plants that can detect lies? It would be a great help during interrogations or investigations.
Well, he might use his reality - controlling power to make himself the most powerful ninja instantly. He could also create a world where ninjutsu has no negative side - effects. For instance, no one would go blind from overusing the Mangekyo Sharingan. Or he could make it so that everyone can understand each other's language without any communication barriers in the ninja world.
One interesting scenario is Harry going back to the time when the Marauders were at Hogwarts. He could interact with his father and his friends, and maybe even learn some new spells from them. Another could be him going forward in time to see what the wizarding world becomes in the future, and then coming back to make changes in the present to avoid a bad future. Also, he might use time control to relive important moments in his life and gain a new perspective on them, like the Triwizard Tournament.
One interesting scenario could be Harry having a conversation with the Whomping Willow. He could find out why it's so aggressive and maybe even convince it to be more friendly. Another could be him chatting with the house - elves through the rats or other small animals that might be in the kitchen. He could learn more about their secret world and their hopes and dreams.
One interesting scenario could be Harry using his space - control ability to instantly transport himself and his friends to hidden, safe places during the Death Eater attacks. For example, he could open a spatial portal to a secret forest glade where they can set up a temporary base. Another could be using time control to go back and prevent certain tragedies, like Sirius's death. He might be able to freeze time right before Bellatrix casts the fatal spell and save Sirius.
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.
Well, the Devil's Snare is quite a fascinating plant in Harry Potter fanfiction. Some fanfictions may depict it as being controllable by dark wizards for malicious purposes. And then there are the Gillyweed plants. In fanfiction, they could be used in underwater competitions or have some sort of magical evolution that allows wizards to stay underwater for even longer periods.
Perhaps a character accidentally transforms into the wrong Animagus form. Say Neville was supposed to transform into a small, unassuming animal like a mouse, but due to a spell mix - up or some magical interference, he turns into a large and clumsy hippogriff. He would then have to deal with not only being in the wrong form but also the chaos that such a large and not - so - subtle form would cause in Hogwarts, like knocking over furniture and scaring the other students.
In some fanfictions, the wedding might be in danger. Maybe Voldemort's remaining followers try to disrupt it. This would lead to an exciting showdown where Harry, Ginny, and their friends have to use their magic skills to protect the wedding. It could also be a chance for Harry to show how far he has come since the end of the war, using his leadership and power to keep everyone safe while still enjoying his big day.
In this type of fanfiction, a possible plot could be that Harry is accidentally sent to an alternate dimension where the rules of magic are completely different. He has to adapt quickly to survive and find a way back home. Along the way, he meets versions of his friends and enemies that are very different from what he knows, which makes for a really exciting and unique story.
One interesting scenario could be them renovating their home at the Burrow. Harry might use his magic to make some unique changes, like creating a hidden room for his memories of his parents. Ginny could be in charge of decorating, adding a Quidditch - themed corner. Another could be their travels. They might visit different wizarding communities around the world, learning new spells and making new friends.