Neville Longbottom could also be a worthy wielder. He has shown great courage and growth throughout the series. With the Grand Sorcerer Staff, he could rise to be a powerful force in the wizarding world, using the staff's power to stand up against any remaining Death Eaters or new dark threats, and also to bring new life and growth to the Hogwarts herbology department which he loves.
One idea could be that the Grand Sorcerer Staff has a mind of its own and it chooses a young, unexpected wizard to be its new master. This wizard then has to learn how to control the powerful magic within the staff while also facing the challenges from dark wizards who want to steal it.
Some fanfictions have the idea that the sorcerer staff can transform into different forms depending on Harry's needs. For example, it can turn into a sword when he's in combat or a key to unlock hidden magical chambers.
A really cool fanfiction is the one that focuses on how the sorcerer staff is the key to an ancient magic that can save the wizarding world from a new, unknown threat. Harry, with the help of his friends, has to unlock the secrets of the staff in order to succeed.
It could be a rogue wizard who wants to steal the power of the Grand Sorcerer. He might be jealous of the Grand Sorcerer's abilities and wants to use that power for his own dark purposes.
Well, perhaps in this fanfiction, the Grand Sorcerer is actually from a different magical dimension and he brings with him new spells and ways of magic. For example, he could introduce a new form of transfiguration that is more powerful but also more dangerous. And he might have a connection to the founders of Hogwarts that no one knew about, which could lead to all sorts of revelations about the school's past and future.
It could be Dumbledore. Dumbledore is always full of surprises and he might give Harry the staff as a last - resort weapon to fight against Voldemort. He could have been secretly researching the staff and its powers and decided that Harry was the only one who could wield it properly.
Well, there are fanfictions that focus on Harry as a super - powerful sorcerer from the very start. Instead of gradually learning magic at Hogwarts, he already has a vast knowledge and can perform advanced spells. Also, there are those that revolve around Harry's love life in a more complex way, such as having a love triangle with Hermione and Ginny, but with a whole new set of magical challenges and adventures thrown in.
Perhaps it's a time - traveling wizard from the future who knows that Harry will need this staff to save the wizarding world at a crucial point. The time - traveler realizes the importance of the staff in Harry's hands and decides to gift it to him. Another possibility is that it's a magical creature in disguise. This creature, having a deep connection to magic and prophecy, presents the staff to Harry as a means of fulfilling a greater purpose in the wizarding world.