Chloe Decker could be one of the main characters. Since she has a special connection with Lucifer in the original story, she might play a crucial role in helping him get his wings back. Maybe she discovers some hidden clue about how to restore his wings.
The stabbing of Clary can completely shift the direction of the fanfiction. It can create alliances that were not there before. Other characters might put aside their differences to find out who stabbed Clary and why. It can also introduce new elements to the story, like a new weapon or a new type of magic that is needed to heal Clary. Plus, it can heighten the tension between different groups, such as the Downworlders and the Shadowhunters, if there is suspicion among them regarding the stabbing.
Another possibility is a character who has been possessed by an evil spirit. They may not be in their right mind when they stab Merlin. This could add an element of mystery as Merlin has to figure out not only who stabbed him but also how to exorcise the spirit from the person's body.
Well, it could lead to a major shift in the story's direction. Maybe Tony's friends would react differently and it could change the whole plot dynamic.
The other characters would likely be in shock. Jace, for example, might be filled with rage and a desperate need for revenge. He would probably blame himself for not being there to protect Clary. Alec might be more practical, immediately trying to find a way to heal Clary. Isabelle would be a mix of worry and anger, angry at whoever dared to harm her friend. Their reactions would also depend on the context of the stabbing within the fanfiction.
Perhaps in the fanfiction, there's a magical object or a powerful spell that can restore his wings. But to find this object or cast the spell, Lucifer has to face many challenges. He might have to team up with some unexpected allies, like a group of angels who initially didn't trust him but later see his determination to get his wings back and decide to help.
One idea could be that Lucifer has his powers stripped by a powerful spell cast by a group of witches. As a result, he's in a vulnerable state and some other characters, perhaps angels who have a bone to pick with him, decide to teach him a lesson in a rather unorthodox way like spanking. It could be a humorous take on the usually intimidating Lucifer.
One possible plot development could be that Naruto's friends rally around him to protect him while he recovers. They might seek revenge on the one who stabbed him.
Maybe it's an enemy from another village who sees Naruto as a threat to their plans. For example, if there's a group in another village that wants to start a war, they might think that getting rid of Naruto would give them an advantage. Another possibility is that it's a character who has been brainwashed or controlled by an evil force. This could add an interesting layer to the story as Naruto's friends would have to not only deal with his injury but also find a way to break the control on the stabber.