Maybe it's a new villain who is trying to make a name for themselves. This villain could see shooting Captain America as a way to gain instant notoriety in the criminal underworld. They might have studied Captain America's weaknesses and found an opportune moment to strike.
It could be an old enemy seeking revenge. Maybe a powerful Titan who escaped Tartarus and blames Percy for his previous defeat. This Titan could have been plotting in secret, waiting for the perfect moment to strike and finally end Percy's life.
Dr. Bruce Banner could play an important role. His scientific knowledge could be used to analyze Captain America's condition and come up with solutions. In some fanfictions, he might even find a way to enhance Captain America's healing abilities. Also, Thor, if he's around, could use his Asgardian magic or knowledge to help in the healing process, perhaps by bringing some rare Asgardian herb or elixir that has healing properties.
It could be an enemy ninja from another village who wants to take out Konoha's strongest shinobi. Maybe someone from the Sound Village still holding a grudge against Naruto.
Well, it could be a rival of Ciel's in business or in a social setting. Maybe someone who is jealous of his status or wealth and sees shooting him as a way to get ahead.
It could be an old enemy of Ezra. Maybe someone from his past who has a grudge against him for some reason, like a business deal gone wrong or a personal feud.
Perhaps it was someone within Toby's own group who had a hidden agenda. A so - called friend who was jealous of Toby or had some ulterior motives that led them to arrange for the shooting. This would add a lot of drama and betrayal to the fanfiction story.
Perhaps a new character who has a hidden agenda. Maybe someone who has a connection to a current case Gibbs is working on and doesn't want him to get too close to the truth. This person might be working for a larger criminal organization and sees Gibbs as a threat.
There's a possibility that it's a family member of a criminal she arrested. They might be seeking justice in their own twisted way. They could have been planning the attack for a long time, waiting for the perfect moment to strike when Burgess was most vulnerable. Another suspect could be a corrupt cop from another precinct who doesn't like the way Burgess and her team are cleaning up the streets and cutting into their illegal activities.
It could be someone from a past case she worked on. Maybe a criminal she put away who has a vendetta against her and managed to get out on parole early. They might see shooting her as a way to get revenge for the time they spent in jail because of her.
Perhaps an enemy from a rival group. They might see Sakura as a threat to their plans, so they poison her to get her out of the way. Another possibility is a character who has been secretly harboring a grudge against her for a long time, and finally decides to take action by poisoning her.