Even Constantine could be a potential teacher. He has a lot of experience with the darker side of magic. He could show Batman how to deal with curses and dark magic that might be used against Gotham. Although their methods might not always align, Constantine's knowledge would be invaluable to Batman in his magic - learning journey.
A new tech - savvy villain who is a master of hacking could be introduced. This villain could pose a threat to both Batmen as they rely on technology in different ways.
In 'batman learns magic fanfiction', perhaps he teams up with magical characters. He could learn from Zatanna, for instance. Their combined skills would make for an exciting story as they take on threats that are both physical and magical in nature. Batman's logical mind might find new ways to apply magic that the magical community hadn't considered before.
Obviously, Alfred is a key ally. He has all that knowledge about Batman's operations and can be a great support to the reborn character. Commissioner Gordon is also an important one. His connection to the police force can be very useful for the reborn Batman in fighting crime.
Don't forget about villains. Cheetah, who has her own connection to the Amazonian lore, could be a great villain in this fanfic. She could be working with some Gotham villains to create chaos in both Themiscyra and Gotham. And perhaps there's a new, original villain created specifically for this fanfic, a power - hungry figure who wants to steal the Amazons' secrets and use them to take over Gotham.
Obviously Batman himself is a key character. Then there could be characters like Catwoman, who has a long - standing relationship with Batman. Wonder Woman could also be part of the harem, given her strong connection to Batman in some storylines. And perhaps Batgirl, as she has a close relationship with Batman in the comics.
Darkseid is another potential foe. He could be interested in the son's unique heritage, wanting to either corrupt him or use his powers for his own evil purposes. Given Darkseid's power, the son would have to rely on both his Batman - like wits and Wonder Woman - like strength to oppose him.
A new character could be a young hacker who initially starts as an adversary to Terry. This hacker could be trying to expose the secrets of Wayne - Powers but then realizes that there are more dangerous forces at work and ends up teaming with Terry. Another could be a genetically - enhanced enforcer working for a new criminal syndicate in Neo - Gotham. He would have superhuman strength and speed, making him a tough opponent for Terry.
A tech - savvy hacker who is initially on the wrong side of the law but later becomes an ally to Batman Beyond. This character could provide valuable information and help Terry with his high - tech battles against villains.
Professor McGonagall could be one of them. She is strict but fair and has a lot of knowledge about transfiguration which could be shared with Harry in a more personal way in this fanfiction.
Commissioner Gordon would likely be a main character. He has been Batman's ally for a long time, and with Batman retiring, he'd have to face the challenges of keeping Gotham safe on his own. Also, Robin, if there is a current Robin in the story, could be a main character. He might be left with the big shoes to fill that Batman has left behind.