Another possible antagonist could be a group of crows. Crows are known to be a bit territorial and aggressive. They might not like the idea of a cat protecting a robin in their area. So they start to harass the mommy cat and the baby robin, trying to drive them out of the place where they live.
Perhaps a stray dog. The dog is always looking for something to chase or bother. When it sees the mommy cat taking care of the baby robin, it decides to make trouble. It tries to scare the cat away from the robin so that it can have fun chasing the small, helpless bird.
One possible plot could be that the mommy cat finds the baby robin fallen from its nest. Instead of seeing it as prey, the cat decides to protect it. She shelters the baby robin from other predators and teaches it some survival skills that a cat uses, like how to be aware of its surroundings. In the end, when the robin is old enough to fly, they part ways but remain friends in a sense.
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.
Maybe the main antagonist is an old enemy of Robin's who becomes jealous of his new powers. This enemy then devises a plan to steal Robin's powers or at least neutralize them so that he can regain his former status as the top threat. He could be a powerful martial artist or a criminal mastermind with a large network of henchmen.
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 a jealous rival. Maybe someone who has always been envious of Robin's abilities or popularity. Now that Robin is in a more vulnerable state as a girl, this rival sees an opportunity to undermine him.
There might be a new, mysterious enemy in the fanfiction. This enemy could be interested in the power that Naruto and his mother possess combined. They try to separate Naruto from his mother and then turn them against each other in order to obtain their power for some evil plan, like taking over the ninja world.
Another vampire family that is jealous of the Mikaelson's power and influence. They see the baby as a threat to their future plans. They might try to kidnap the baby and use it as a bargaining chip to gain more power in the vampire world. They would be constantly plotting against the Mikaelsons to get their hands on the baby.
Danzo could be a great antagonist even when Naruto is a baby. He might see Naruto as a threat to his plans for Konoha's future and try to get rid of him secretly while making it look like an accident.
It could be a new villain who has a grudge against the Justice League. Maybe someone whose family was affected by the League's actions in the past. This villain would be very smart and powerful, creating elaborate traps for the League members.
Perhaps a powerful sorceress. She could be jealous of Robin's connection to Batman and the other heroes, so she uses her magic to turn Robin into a girl as a way to humiliate and weaken him. The sorceress could then try to use Robin's new form to her advantage, like trying to make Robin turn against the heroes or something.