Perhaps Ra's al Ghul. He has his own plans for Gotham, and Robin hiding in Batman's cape could be part of a larger scheme to either protect a secret from Ra's or to gain an advantage over him. Ra's al Ghul's intelligence and long - term planning would make him a formidable antagonist in this fanfiction.
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.
Well, perhaps Robin is trying to avoid a villain and sees Batman's cape as the perfect hiding spot. It could be during a fight in a crowded alley, and Robin slips under the cape to surprise the enemy later.
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.
It could be someone who is jealous of Naruto's hidden genius. Maybe a fellow ninja who senses that Naruto is hiding something and wants to expose him out of envy. This person might try to undermine Naruto at every turn.
It could be Voldemort. Even if he was defeated in the original story, in this fanfiction he might have some sort of lingering influence or perhaps a follower who is continuing his dark work. Harry hides behind the mask to avoid being detected by this new threat related to Voldemort.
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 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.
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.
A new dragon - hunter could be the main antagonist. He might be extremely skilled and ruthless, and Hiccup hides his strength to better understand this new threat and find a way to defeat him without putting his village in immediate danger.