Ra's al Ghul could also be a main villain. He might see the absence of Robin as a sign of weakness in Batman's legacy. He could try to take advantage of this and further his own plans for Gotham, perhaps even trying to recruit Robin for his own purposes if Robin is out on his own.
Black Mask could become the main villain. Without Robin to add a different dynamic to Batman's crime - fighting, Black Mask could rise in power more easily. He could take over more of Gotham's underworld, and his activities could be more unchecked as Batman is dealing with the absence of Robin in his life, which might have some psychological impact on how he approaches crime - fighting.
Ink could be a great main villain. He's a character from the Batman Beyond series known for his ability to manipulate ink - like substances. He could target Robin specifically, seeing him as a weak link in the Batman - Robin duo. His powers would pose a unique challenge for Robin to overcome.
Another storyline could be that Robin is adopted by another superhero or a group. For instance, he could be taken in by the Titans early on. This would change his character development as he'd be influenced by different people and their ideologies. He might develop a more team - based approach rather than the mentorship model that would have occurred with Batman.
There might also be other characters from the Batman universe involved. For instance, Alfred, Batman's loyal butler. He could play a role in trying to mediate between Batman and Robin when things go wrong. Another character could be one of Batman's villains, like the Joker, who might take advantage of the situation between Batman and Robin.
One main theme is often loyalty. Robin is loyal to Batman and vice versa. They rely on each other in the fight against crime in Gotham. Another theme can be growth. Robin is constantly growing as a hero, learning from Batman's experience.
Lex Luthor would be a great choice. He could be plotting against both Batman and Supergirl. He might try to use his intellect to create a weapon that can nullify Supergirl's powers and outwit Batman's strategies at the same time.
Kingpin would be a very suitable main villain. He has his criminal empire in Hell's Kitchen already, and if he decides to make a move on Gotham, it would bring him into conflict with Batman. His power, wealth, and ruthlessness would make him a formidable foe for the two heroes to face together.
The Joker would be a great choice. He could use some new - fangled chemical or device to mind control Batman. Imagine the chaos he could create with a mind - controlled Dark Knight at his beck and call. He might make Batman do really out - of - character things like laughing hysterically while committing crimes.
Another option could be a new villain created specifically for the fanfic. This villain could be someone who has a vendetta against the Wayne family because of something Batman's dad did in the past. He could be a master of disguise and manipulation, making it difficult for Batman and his dad to figure out who he really is.
The most well-known villain of Batman in the comics is the Joker. He's a chaotic and unpredictable force that constantly challenges Batman.
The Penguin could be a great addition. His wacky and devious schemes always make for an interesting story. Also, Catwoman. Her complex relationship with Batman adds a layer of intrigue. And Oracle, who can provide valuable information and support from afar.