It could be a new villain, someone who is jealous of Robin's ability to turn the city into his jungle gym. This villain creates devices that disrupt Robin's movements, like magnetic fields that prevent him from using metal poles or grappling hooks. Robin has to find a way to overcome these disruptions and defeat this envious villain.
The Riddler could be a great antagonist. He could set up puzzles and traps all over the city that is now Robin's jungle gym. Robin would have to solve the riddles to avoid the traps while still using the urban jungle gym to his advantage.
Perhaps Poison Ivy. She could use her plant - based powers to transform parts of the city into a more literal jungle gym, with vines and overgrown plants everywhere. But these plants are dangerous and she wants to stop Robin from using them as his own playground. Robin would have to figure out how to deal with her and still use the new, plant - filled environment as his jungle gym.
In this fanfiction, a plot could involve Robin challenging Batman to a friendly competition in using the city as a jungle gym. They set up various challenges like who can get from point A to point B the fastest by only using rooftops, poles, and other urban structures as if they were jungle gym equipment. This would show their different approaches to using their environment for movement and also their relationship in a fun, competitive light.
The main antagonist could be a deranged chemist who was once shunned by the scientific community. He uses his knowledge of chemistry to create deadly toxins and gases to get revenge on those who wronged him. He sees Batman and Robin as obstacles in his path of destruction.
Lex Luthor could be a great main antagonist. He could be jealous of the trio's popularity and see them as a threat to his own power - hungry plans. He might create elaborate traps and use his vast resources and intellect to try and take them down, all while trying to make it look like they are the bad guys in the public eye.
The most obvious antagonist could be the Joker. He loves to torment Batman, and kidnapping Robin would be a great way to do that. He might have some elaborate plan to break Batman's spirit by harming Robin.
How about Ra's al Ghul? He has his own ideas about justice and order that are very different from Batman's. He could see Robin as a potential recruit for his League of Shadows. Ra's al Ghul could manipulate events to make Robin question Batman's methods and try to draw Robin to his side.
A new, self - made villain who is obsessed with colors and wants to collect all the 'powerful' colors in the world. Robin's red stands out to him as one of the most important ones to obtain, so he sets out to defeat Robin and take his red essence.
The Batman Who Laughs could also serve as the main antagonist. He represents a twisted version of Batman, and his goal could be to infect all the Batverses with his madness. His knowledge of Batman's tactics and his own insanity would pose a huge challenge for the normal Batman as he tries to stop him from spreading chaos.
The Joker could be a likely main antagonist. He might be the one who manipulates the situation to get Batman arrested, perhaps by framing him for one of his own heinous crimes. The Joker loves to play mind games, and getting Batman locked up would be a huge part of that.
The Joker is always a great choice. He could come up with a particularly cruel and elaborate plan to put Batman in peril, like kidnapping all of Batman's allies and forcing him to choose which ones to save.
The Joker could be a likely antagonist. He might be causing chaos in Gotham, and Robin has to hide in Batman's cape to avoid being spotted by him. The Joker's unpredictable nature would add tension to the story.