There's also a possibility that it could be a disgruntled former colleague or someone who feels wronged by Gibbs in some way. It could be a person who was fired from NCIS because of Gibbs and now wants to get back at him.
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.
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 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.
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 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.
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.
One possible plot twist could be that the shooter has a personal vendetta against Gibbs that dates back to a case that Gibbs thought was long closed. Maybe the shooter was wrongly accused in that case and has been planning revenge ever since.
It could be a former criminal that McGee helped put away. They might have escaped from prison and are seeking revenge on him for ruining their life.