There might be. A lot of fanfic writers are inspired by different concepts. However, it could be a bit niche, so you may need to do a thorough search on various fanfic websites. Check the tags related to 'Percy' and 'primordials' on those platforms.
In fanfiction, if Percy is trained by the Primordials, it's a really interesting concept. The Primordials are like the originators of many things in the fictional world. Their training might involve Percy learning ancient secrets, perhaps how to control primordial elements such as chaos or creation energy. This could lead to Percy becoming a very different kind of hero, with powers and knowledge that set him apart from his normal self in the original stories.
Perhaps the fanfic focuses on Percy's relationship with the god he is a champion for. There could be internal conflicts as he tries to balance his own values with the demands of the god. He might also have to interact with other demigods who are either jealous or in awe of his new status.
A theme that can't be ignored is the theme of friendship. Even though he is the champion of Hera, his friendships still play a huge role. His friends might support him, question his actions as the champion, or even be in danger because of his new role. This creates a complex dynamic that is often explored in these fanfics. Also, the theme of destiny. Percy has to figure out if being the champion of Hera is part of his true destiny or just a new path he has to tread.
One popular Percy Jackson champion fanfic could be 'The Ascent of the Champion'. It might focus on a new adventure where Percy has to prove himself as a true champion in a different realm. Another could be 'Champion's Trial' which perhaps puts Percy through a series of difficult challenges to earn the title of champion in a more ancient and mysterious context. There could also be 'Percy, the Unlikely Champion' which might explore Percy's journey to becoming a champion in an unexpected way.
Percy himself is always interesting. In fanfics, he might be even more complex as a champion. Another interesting character could be Annabeth. In some champion - related fanfics, she could be the strategist behind Percy's success as a champion. And then there's Nico. His dark powers and his own journey could intersect with Percy's in interesting ways in these fanfics when Percy is in his champion role.
A powerful mythological creature that has a grudge against the god Percy is championing for. For example, a Titan who was once defeated by that god and now sees Percy as a way to get revenge. It would be a great adversary, with its own set of powers and minions.
One possible plot for a 'Percy Jackson Champion of Nyx fanfic' would be that Nyx chooses Percy as her champion to stop an ancient evil that is rising. Percy would have to train in the ways of Nyx, which are very different from what he's used to. He'd have to learn to harness the power of the night, deal with Nyx's shadowy minions, and find a way to unite the demigods who are skeptical of his alliance with Nyx. As he delves deeper into this role, he uncovers secrets about Nyx's past and how it ties to the future of the demigod world.
A simple plot could be a race against time. They have to reach a certain place before an evil force does. Thalia could use her speed and strength, Percy could use his strategic thinking as a champion, and Zoe could use her knowledge of the terrain to lead them. They might encounter traps and minions of the evil force along the way.