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.
Another possibility is that it's related to a new case she was just starting to investigate. There could be a corrupt figure within an organization she was looking into, and they decided to take her out before she could dig deeper. This suspect would likely be more sophisticated and have the resources to cover their tracks.
Perhaps it's a personal enemy, not related to her police work at all. For example, someone she had a feud with in her personal life who saw an opportunity to harm her when she was on duty. This would add an interesting layer of complexity to the fanfiction as it blurs the lines between her personal and professional life.
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.
Perhaps a mysterious stranger who has some unknown vendetta against the Heartland family. This person could have been lurking around the ranch for a while, waiting for the perfect opportunity to strike. Or it could be someone within the community who has a hidden agenda and sees Amy as an obstacle to their plans.
Well, perhaps a new character introduced in the fanfiction. Maybe a spy from a rival agency who sees Abby as a threat to their mission. Or it could be an associate of a major criminal that the NCIS has been investigating. They might think that by shooting Abby, they can disrupt the investigation. Also, it's possible that it's a person with a personal vendetta against Abby, like someone she unknowingly wronged in the past.
It could be an ex - boyfriend who is jealous of Annie's new relationships. Maybe he couldn't handle seeing her move on.
It could be an old enemy of Chuck's. Maybe someone from his past who has a vendetta against him for something he did before.
Another way is that she could be hurt while undercover. If she is infiltrating a dangerous gang, they might discover her true identity and attack her. This could lead to some serious injuries, like being beaten up or even shot.
It could be an old enemy of Nicole's from her police work. Maybe someone she put in jail before who has a vendetta against her. This person might have been released recently and is out for revenge.