Maybe an ancient evil spirit that has a vendetta against Ash. The spirit could possess the aura guardian and make it turn against Ash. In this scenario, the main antagonist is not really the guardian but the spirit that controls it, and the story would be about Ash fighting this spiritual evil.
There could be a group of poachers or thieves who want to capture the aura guardian. They trick the guardian into thinking that Ash will lead them to it, so the guardian betrays Ash in an attempt to protect itself. However, as the story unfolds, the guardian realizes it was deceived and teams up with Ash to fight the real villains.
Ash himself could be the main antagonist as he is the one doing the betraying.
It could be that there is a young, impressionable character who looked up to Ash Aura. After the betrayal, this character is confused and starts to question his own beliefs. There might also be a powerful mage in the story who has the ability to see the truth behind Ash Aura's actions and is trying to prevent the chaos that the betrayal has caused.
It could be a powerful and jealous Galar trainer. This trainer might see Ash as a threat to their own reputation and success in the region. They would then use their influence and charm to turn Ash's friends against him.
Perhaps the aura guardian was mind - controlled. An evil force took over its mind and made it act against Ash. In this case, the story could revolve around Ash trying to break the mind - control and save his friend. It would involve a lot of action and mystery as Ash figures out what's wrong and how to fix it.
A new villain could be introduced. For example, a powerful trainer who is jealous of Ash's new powers. This trainer wants to defeat Ash and steal his aura psychic abilities to become the most powerful trainer in the world.
It could be a new and powerful trainer who manipulates Anabel into betraying Ash. This trainer might be jealous of Ash's success and see Anabel as a means to bring Ash down.
A powerful and envious Pokémon trainer could be the main antagonist. This trainer might be jealous of Ash's bond with Greninja and decides to break it. He could use dark magic or some sort of mind - controlling device to make Greninja turn against Ash.
It could be a new character who is jealous of Ash and Skyla's friendship. This character might spread false rumors about Ash to Skyla, causing her to feel betrayed. So, this new jealous character would be the main antagonist.
The Pokemon Professor in the region could also be the antagonist. Suppose the professor discovers a rare and powerful Pokemon that Ash is also interested in. The professor, driven by greed or a desire for power, might betray Ash by making it seem like Ash is trying to steal the Pokemon, causing all of Ash's friends and the community to turn against him.
Maybe there's a power - hungry rival from Ash's own group who becomes jealous of his newfound ability and turns against him. This rival could then become the main antagonist, constantly trying to undermine Ash and take his power.