In some fanfictions, Sirius Black could be a target of Snape's revenge. Sirius played pranks on Snape at school and was part of the group that bullied him. Snape may see Sirius as an accomplice to James Potter's actions. So, he might plan some elaborate revenge schemes against Sirius to settle old scores.
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.
The Death Eaters who have caused harm to Harry's family and friends are also probable targets. Snape, having been a double - agent among the Death Eaters, could help Harry take revenge on those who have killed his parents or endangered his loved ones. He could provide crucial information about their whereabouts or weaknesses.
Often, the theme of revenge in Severus Snape fanfiction is related to love. Maybe he's seeking revenge for Lily's death. He could be plotting against the wizarding world that he feels failed to protect her. This can lead to some really intense and emotional storylines where his love for Lily drives his need for vengeance in ways that are not always straightforward.
Bellatrix Lestrange could also be a character who interacts with Snape. If Snape goes back far enough, he might try to influence Bellatrix's path and keep her from becoming such a devoted Death Eater. This could have a major impact on Voldemort's power and the overall balance of the wizarding world.
I think Hisoka could also be involved in tickle revenge in some strange and twisted fanfictions. Hisoka has his own unique sense of 'fun' and if someone were to tickle him as a way to mock or play with him, he would surely retaliate in a very Hisoka - like way. Also, Illumi might be involved if it's related to his brother Killua. If someone were to tickle Killua in a way that Illumi thought was inappropriate or dangerous, he might plan a tickle revenge on behalf of his brother.
The Akatsuki could also be antagonists in Uzumaki clan revenge fanfiction. They were after powerful ninja and the Uzumaki clan was full of power. They could have been the ones who initiated the attack on the Uzumaki clan to get their hands on things like the Uzumaki's powerful chakra or their sealing techniques. And then there might be a secret group within the Leaf Village itself that was involved in the Uzumaki clan's downfall, perhaps out of fear of their growing influence.
Harry might start by making a list of all the wrongs Snape has done to him over the years. This would include things like unfair grading, public humiliation, and general nastiness. Then, he could look for ways to expose these actions to the rest of the wizarding world. Maybe he would use the wizarding newspapers or even start his own blog (if there were such a thing in the wizarding world) to share his experiences. He could also try to find allies among the students and faculty who have also had problems with Snape.
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.
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.
Obviously, Harry Potter himself is the central figure. His friends like Ron and Hermione would also be involved. They would support Harry in his revenge plans, with Ron providing his loyalty and Hermione using her intelligence to strategize. Ginny Weasley might also be part of it, as she is Harry's love interest and a strong - willed witch in her own right.
Sasuke could be one of the characters to support Naruto. Given their complex relationship and Sasuke's own issues with Konoha in the past, he might see Naruto's cause as just. Another one could be Jiraiya. If he were alive and found out about the real reasons behind Naruto's revenge, he might side with Naruto as he always cared for him deeply. Also, some of the outcast ninja within Konoha, like those who have been discriminated against because of their bloodline or lack of skills, might support Naruto as they also have grudges against the village system.