Nightwing could also be a likely savior. He was once Robin himself and has a strong bond with the current Robin. He might take it upon himself to rescue Robin, using his own unique fighting style and experience to outwit the Joker. Plus, he has his own network of contacts in Gotham that could help in the search.
Batman, of course. He's Robin's mentor and protector. He has the skills, the resources, and the determination to track down Slade and rescue Robin.
In one scenario, the Joker might kidnap Robin to taunt Batman. He could hold Robin in a secret, elaborately booby - trapped lair. Robin, being the resourceful sidekick, would try to find ways to escape and leave clues for Batman. The Joker would then play mind games with both Robin and Batman through recorded messages and false leads.
His teammates are very likely to be the ones to save him. Aqualad, with his leadership skills and water - based powers, could find a way to get Robin out of a dangerous situation. He might use his strategic mind to outwit the threat and rescue Robin.
The plot might go like this. Slade kidnaps Robin in a very stealthy way. He takes Robin to an abandoned building. There, he tries to indoctrinate Robin into his way of thinking. He could be telling Robin that Batman is using him and that he can offer Robin more power. However, Robin stays true to his beliefs. Meanwhile, Batman is on a frantic search for Robin, following every lead, and the Bat - family is also on high alert. Robin is constantly looking for an opportunity to turn the tables on Slade and escape.
Robin would probably be both scared and angry. He's a brave kid, but facing the Joker alone is no small feat. He might initially try to fight back if he has the chance.
One possibility is that Robin, being kidnapped, will have to rely on his own wits and skills he has learned from Batman. He might try to outsmart the Joker by finding weak points in the place where he is held captive. For example, if it's an old warehouse, he could look for ways to escape through the ventilation system or by using some tools he might find around. And the Joker, being the unpredictable villain, could be constantly taunting Robin, trying to break his spirit before Batman arrives.
One possible fanfiction plot could be that Deathstroke kidnaps Robin to use him as bait for Batman. He wants to lure Batman into a trap and finally defeat him. Robin, being the resourceful sidekick, leaves behind small clues for Batman to find him, adding an element of mystery and suspense to the story.
In a different take, a new character could be introduced as the main antagonist. Maybe a criminal mastermind who teams up with the Joker specifically to target Robin. This new character could have a personal vendetta against Robin or see him as a key to achieving some greater evil plan, and the Joker is just a means to an end in this scenario.
A new super - villain group that wants to take over the world might be behind it. They figure that by experimenting on Robin, they can learn the secrets of the Young Justice and use that knowledge against them. Or perhaps an old enemy of Batman who wants to get back at him by harming his protege.
Robin does not become the Joker. The characteristics and roles of these two characters are completely different. They exist within separate story arcs and have no direct transformation connection.