A great plot could involve Harry and Voldemort's duel taking place in an alternate dimension. In this dimension, the rules of magic are different. Harry has to quickly adapt while Voldemort struggles because his reliance on dark magic doesn't work as well here. Or perhaps Harry is joined by the spirits of his parents and other loved ones who died because of Voldemort. Their presence gives Harry strength and new magical abilities that he uses to outwit Voldemort.
Well, it could be that the duel is not just a physical battle of spells. Instead, it becomes a battle of wits and ideology. Harry tries to convince Voldemort to abandon his path of darkness by showing him the consequences of his actions. Through a series of magical projections, Harry shows Voldemort the pain and destruction he has caused. This causes Voldemort to hesitate during the duel, giving Harry an opening to defeat him.
One interesting plot idea could be that Harry discovers a new and ancient magic that gives him an unexpected edge in the duel. For example, he might find a spell that can temporarily freeze Voldemort's dark magic, allowing Harry to strike. Another could be that Harry has a vision during the duel that shows him the true source of Voldemort's power, and he uses this knowledge to target it directly. Maybe it's a hidden object or a bond with a dark entity that Harry manages to sever, weakening Voldemort.
One idea could be that during the duel, Harry discovers a hidden spell that Snape was about to use against Voldemort. So instead of a full - on fight, they end up in a truce to discuss this new revelation. This could lead to a whole new understanding between them.
One idea could be a new type of spell being introduced in the dueling circuit that throws everyone off balance. Maybe a spell that can temporarily freeze an opponent's wand. Another could be a character from an unexpected house rising to the top of the dueling circuit rankings, like a Hufflepuff who has some unique dueling techniques.
One idea could be that Harry discovers a new spell that can penetrate Voldemort's dark magic defenses and uses it to shoot him. Another could be that Harry teams up with some unexpected allies, like the house - elves, who help him get in a position to shoot Voldemort. Maybe Harry has to sacrifice something important, like his invisibility cloak, to gain the power needed to fire the shot that will defeat Voldemort.
One idea could be a story where Bellatrix has a moment of doubt about Voldemort's methods. Maybe she starts to see the destruction he's causing and has to decide whether to stay loyal or turn against him. And Harry Potter could somehow be involved in this internal struggle of hers, perhaps by showing her a different path through his own actions and values.
Maybe Harry absorbs Voldemort and then has to deal with the internal struggle of Voldemort's thoughts within his mind. He constantly battles Voldemort's voice, which tries to persuade him to use power in a wrong way. But Harry uses his own strong will and the love he has for his friends and family to overcome it.
One plot idea could be that the female Voldemort has a change of heart. Maybe she starts to see the error of Voldemort's ways after interacting with Harry Potter. She could start to secretly help Harry and his friends against the Death Eaters, which would create a lot of internal conflict within the Dark side.
One idea could be a fanfiction where Voldemort has a change of heart due to some ancient magic. Maybe he discovers a spell that shows him the true consequences of his actions and decides to work with Harry to fix the wizarding world instead of destroying it.
One idea could be that Harry, influenced by dark magic, forms an uneasy alliance with Voldemort. They could team up to take over the wizarding world in a different way than Voldemort originally planned, perhaps by ruling jointly. Another plot could involve Harry exploring the darker aspects of his own magic and using some of Voldemort's knowledge to do so, which causes a rift between him and his friends.
One idea could be a fanfiction where Voldemort gets a second chance at life but without his dark magic powers. He has to learn to live as a normal wizard in the wizarding world and Harry Potter has to decide whether to trust him or not.
One plot idea could be that Moaning Myrtle was actually a target of Voldemort's early experiments with dark magic. Maybe he tried to use her as a test subject for some sort of soul - splitting magic before he fully developed the concept of Horcruxes. This would give Myrtle a much more significant role in the overall 'Harry Potter' lore and explain some of her more erratic behavior.