The other Hogwarts professors like McGonagall and Flitwick might be included. If Snape is getting revenge on students who misbehaved towards him, characters like Draco Malfoy or the Weasley twins could be part of the story. Draco might be involved if he had previously mocked Snape, and the Weasley twins could be targets if they had played pranks on Snape that he now wants to pay back.
One possible plotline could be Snape getting revenge on those who bullied him during his school days. He might use his potions skills to create some harmless but embarrassing concoctions for them.
In some versions, Snape might use his knowledge of the Dark Arts to expose the wrongdoings of those who have wronged Harry. For example, if there are Death Eaters or bullies in Hogwarts who target Harry, Snape could reveal their secret plans or actions that are against the school rules or the wizarding law.
Hermione could be the sender. If Snape has made some unfair judgment in class against Gryffindor students, Hermione, with her strong sense of justice, might send a howler to him to voice her protest.
One common way for Marinette to get revenge in fanfiction could be through her fashion skills. She could design something that makes Lila look foolish. Like creating a dress for an event that has a hidden message or design that mocks Lila's lies when she wears it. It's a more creative and less confrontational way of getting revenge, but still very effective in showing Lila that Marinette is not one to be trifled with.
Snape could face some backlash from the Ministry. If they find out that he was involved in getting rid of Umbridge, they might try to find ways to punish him or at least keep a closer eye on him. However, his skills at subterfuge would likely help him avoid any serious consequences.
The Death Eaters could be targets too. They might have looked down on Snape at times or made his life difficult within the Dark Lord's ranks. Snape could use his knowledge of their secrets and weaknesses to take revenge. Also, in some fanfictions, Dumbledore could be a target. If Snape feels that Dumbledore used him or lied to him, he might seek revenge on the old wizard, although this is a less common but very interesting take.
In some fanfictions, Harry might expose Snape's past wrongdoings to the whole school. For example, if Snape had been unfair in grading or showing favoritism to certain students, Harry could find evidence and make it public. This would damage Snape's reputation among the students and teachers.
It could be a rogue ninja who has a personal vendetta against Boruto's family. This ninja might have caused harm to Boruto's father Naruto in the past, and now Boruto is determined to make him pay.
Perhaps the target is Howard. Howard might have made a joke about Sheldon's mother that Sheldon didn't appreciate. As a result, Sheldon could try to get revenge by interfering with Howard's work on the space station projects, causing minor but annoying setbacks.