It could be a rival trainer who has feelings for May and tries to drive a wedge between her and Ash. This rival might spread false rumors about Ash to make May doubt him, leading to Ash's heartbreak.
Well, perhaps May decides to pursue her own Pokémon journey alone, leaving Ash behind. This could break Ash's heart as he might have developed deep feelings for her during their adventures together. Maybe she meets a new trainer who she seems more interested in, and Ash feels replaced.
Maybe a rogue Pokémon trainer who has a grudge against Ash. This trainer has dabbled in some dark and forbidden Pokémon training techniques that allow them to control minds. They target Ash because he represents everything they are not - kind, successful, and loved by many Pokémon.
The main antagonist could be a rogue Pokémon trainer who has a special Hypno that has been enhanced to have this 'L to obey' power. This trainer might want to use this power to take over Pokémon battles and tournaments, so Ash has to stop him.
A powerful and greedy Pokémon collector could be the main antagonist. He wants to capture all the rare Pokémon in Unova for his own collection, regardless of the consequences for the Pokémon or the ecosystem. He sees Ash and May as obstacles in his path, especially when they try to protect the Pokémon they encounter.
One possibility could be a trainer who is jealous of Ash's new - found muscle - powered abilities. This trainer might see Ash as a threat to their own status in the Pokémon world. They could try to undermine Ash by using underhanded tactics during battles, like trying to tire him out before the fight even starts.
His rival, Gary, could be a likely candidate. Gary has always been competitive with Ash, and in this fanfiction, his jealousy could reach a boiling point, causing him to betray Ash for his own gain, like trying to steal his Pokémon or get credit for his achievements.
One great 'Ash Ketchum May Fanfiction' is 'A Journey Together'. It beautifully depicts their adventures in a new region, with great character development for both Ash and May. The story has a perfect blend of action, friendship, and a hint of romance.
It could be a famous Pokémon trainer. Maybe someone who has traveled to many different regions and has a lot of knowledge about rare Pokémon.
His rivals could be the ones to betray him. For example, Gary Oak might see Ash's godlike powers as a threat to his own status and decide to plot against him. Gary could spread false rumors about Ash to turn others against him.
A simple plot could be that they are exploring an ancient Pokémon ruin. May accidentally steps on a hidden mechanism and is about to fall into a pit. Ash quickly grabs her and then, realizing she's a bit shaken, carries her bridal - style out of the ruin. This event makes them closer as they share a moment of vulnerability.