Another character could be Terry's love interest. She could play a role in the twins' story, perhaps she has a connection to their past or she gets caught in the middle of their conflict with Batman Beyond, making the story more complex and engaging.
There could be some new characters who are splicers themselves. Maybe a lead splicer character who is trying to create a new world order for the splicers. This character could be complex, having both good intentions for his splicer kind but going about it in the wrong way, like causing chaos in Gotham.
Terry McGinnis is bound to be a main character. He is Batman in this future setting. His alter - ego as a young man trying to balance his normal life and his duties as Batman would be central to the story.
Perhaps an old enemy of Batman who has made a deal with Cadmus. They combine their knowledge and resources. This could be someone like Mr. Freeze, who has found a way to use Cadmus' genetic engineering to make himself even more powerful. His new form and abilities would pose a great challenge for Batman Beyond.
Ink could be a great antagonist. He has the ability to manipulate his body into various weapons and forms. He could be seeking to take over Gotham's underworld in the 'Batman Beyond' era and sees Nightwing and Batman Beyond as threats to his plans.
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.
Robin, in all his incarnations, is also a popular character in fanfiction related to Batman. He represents the sidekick aspect and the growth and development that comes with it. In Batman Beyond, Commissioner Barbara Gordon also features prominently in some fanfictions. Her connection to the original Batman era and her role in the new one can be explored in interesting ways.
Well, Batman himself is likely to be one of the main characters. He would be the one trying to deal with the situation involving the splicers. There might also be some new characters created specifically for this fanfic that are related to the splicers, perhaps a leader among them or a scientist who created them.
Bruce Wayne is also important. Even though he's older, he still plays a significant role. He mentors Terry and provides him with the knowledge and resources to be Batman. Their relationship is a key aspect of the story. Additionally, characters like Max could also be main characters. She's Terry's friend and often gets involved in his adventures, sometimes helping him out with her hacking skills and general intelligence.
Brother Eye could be a possible main antagonist. If it were to gain the ability to time - travel, it could try to rewrite history to create a world where it has complete control. It could manipulate events in different eras to eliminate any threats to its dominance, and Batman Beyond would have to stop it from achieving its goal.
Sometimes other villains from the Batman Beyond universe might make appearances in the fanfiction related to Blight. They could either team up with Blight, adding more challenges for Batman, or they could be involved in some sort of power struggle within the criminal underworld that is affected by Blight's actions.