The Joker could still be a major villain. Even with Batman as a god, the Joker might find some twisted way to challenge him. Maybe he'd try to drive Batman to use his god - like powers in a way that goes against his moral code. Ra's al Ghul could also be a villain. He might see Batman's new godhood as a perversion of the natural order and seek to destroy him.
Well, in 'batman becomes a god fanfiction', the Court of Owls could be a really interesting villain group. They might see Batman's ascension to godhood as a disruption of their long - standing plans for Gotham. They could try to use their ancient knowledge and resources to undermine Batman. Also, Bane could be a villain again. He might think that Batman, with his new powers, has become too powerful and decide to break him once more, believing that no one should have that much power over others.
Lex Luthor could be a likely villain. He's always devising plans to take down the Justice League, and this time he might have come up with a scheme that almost succeeds, until Batman steps in. For instance, he could create a super - weapon that targets the unique powers of each Justice League member.
The Joker could be a likely villain. Imagine his chaotic nature combined with the supernatural. He might team up with a demon to create even more mayhem in both Gotham and Sunnydale. His unpredictable actions would pose a huge challenge to Buffy and Batman.
Obviously, Nightwing himself would be the main character as he's the one taking on the Batman role. Robin, if there's a younger Robin at the time in the fanfiction, would also be a key character. He might look up to Nightwing in his new role or have some doubts about him filling Batman's shoes.
Darkseid is another possible villain. He is extremely powerful. He could send his minions to attack the Justice League first, weakening them, and then use some sort of magic or technology to trap them. Batman would then have to find a way to free the League from Darkseid's clutches, perhaps by finding a weakness in Darkseid's power source.
One likely villain could be Loki. He's always up to no good and loves to play mind games, so a card game would be right up his alley. He might be the one who lures Tony into the game.
The Joker could be an antagonist even when Batman becomes a god. He might see Batman's new status as a challenge and try to bring him down to his level through his usual chaotic and insane methods. Maybe he'd try to drive Batman mad with the idea that his new powers make him no better than the villains he fights.
Annabeth would most likely be his ally. She has been with him through thick and thin, and her wisdom would be invaluable as Percy becomes a god. Grover could also be an ally, using his nature - related powers to help Percy in different situations. Another possible ally could be Nico di Angelo. With his control over the dead, he could provide unique assistance, especially when it comes to dealing with the darker aspects of the godly world.
Orochimaru could be like the Joker. He's already a bit of a mad scientist and likes to play with people's lives. Just as the Joker likes to create chaos, Orochimaru could do the same in Konoha, and Naruto, as the Batman - inspired hero, would have to stop him. He could also have some henchmen like the Joker does.
Obviously Percy himself. Also, Annabeth would probably be there as his girlfriend. Grover, his satyr friend, might also make an appearance. And the gods, like Poseidon, Percy's father, would surely play a role, either guiding Percy or testing him on his path to godhood.
DIO could be a likely villain. Since he has a long - standing feud with the Joestar family. He might try to take advantage of the situation where Jotaro and Jonathan meet to cause chaos and try to defeat them both.