It could be Dumbledore, even if he's passed away. His portrait or some kind of magical remnant of him might still be able to guide Harry in learning new magic.
Well, he might discover a long - lost spell. Maybe it's a spell that can make inanimate objects come to life and help him in his daily tasks at Hogwarts.
It might be a story where Harry delves into the family magic in order to solve some sort of mystery. For example, he could be looking for a way to defeat a new dark force and realizes that the family magic holds the key. He could learn new forms of magic related to protection or divination that are unique to his family.
One main theme is self - discovery. Harry realizing his ability for wandless magic is like finding a new part of himself. Another theme is breaking boundaries. Since wandless magic is not as common in the wizarding world shown in the original books, it represents going beyond the normal magical practices. And of course, there's the theme of growth, as Harry has to learn and develop this new skill, which often comes with challenges and setbacks.
Well, it could be that Harry has an encounter with an Egyptian wizard who decides to teach him their unique brand of magic. This magic might involve things like controlling the sands or communicating with mummies. Through his learning, he gains new powers and uses them to face his enemies in a different way than before.
One interesting plotline could be Harry discovering a new and ancient form of magic that was lost for centuries. This could lead him on a journey to understand its origin and how to master it.
One popular 'harry learns wandless magic fanfic' is 'The Wandless Wizard'. It has an interesting plot where Harry discovers his latent ability for wandless magic through a series of challenging adventures. Another one could be 'Wandless Magic: Harry's New Path', which focuses on how Harry's journey in learning wandless magic changes his relationships with his friends and teachers at Hogwarts.
Some of the teachers at Hogwarts could be involved as well. Professor McGonagall, for instance, might teach Harry some advanced Transfiguration magic. And Snape, before his death, could have left behind some notes or knowledge about Potions - related magic that Harry discovers and uses to learn more powerful potions and the magic associated with them.