It could be an enemy dragon. There are many rival dragons in the Fairy Tail universe, and if they see Natsu with Igneel, they might target Natsu to get to Igneel. Dragons can be very territorial and protective of their own, and an enemy dragon may see Natsu as a weak point to attack Igneel through.
Maybe it was an accident during their training. Igneel might be teaching Natsu a new and powerful dragon - slaying technique. Since these techniques are often very dangerous, Natsu could have misjudged a move and ended up getting hurt. Igneel would feel guilty, but at the same time, it could be a valuable learning experience for Natsu.
One possibility is that Igneel would immediately go into a protective mode. He might use his dragon powers to heal Natsu if he has such abilities. Since they have a strong bond, Igneel would be enraged at whatever caused Natsu to get hurt and would seek revenge. He could also stay by Natsu's side until he recovers, providing warmth and comfort as dragons are often associated with being a source of strength and protection.
One of the senior and more experienced members of the guild. They have seen a lot and know how things should be done in the guild. If Natsu acts in a way that goes against their long - established ways, they would not hesitate to berate him.
There's also a chance that an old enemy of the Thundermans, who has been lying low, resurfaces and decides to take revenge on the family by hurting Max. This enemy might have some new powers or a new plan that catches Max off - guard and leads to his injury.
A supernatural hunter might be responsible. Hunters in Teen Wolf sometimes misjudge humans as being involved with the supernatural. They could have thought Danny was a part of something bad and injured him while trying to 'deal' with the situation.
In some fanfictions, it could be Gray. Their rivalry - turned - friendship has a lot of potential for such an unexpected plot twist where Gray is the father.
Maybe it's a new villain in the fanfiction. Some powerful and evil character who sees Leo as a threat to their plans. This villain could use their unique powers or weapons to harm Leo. For example, a villain with the ability to control electricity could zap Leo and cause internal injuries.
It could be a rival from another house. For example, a jealous Slytherin who wants to take Ginny out of the Quidditch game. They might use some underhanded tactics to make her broom malfunction and cause her to get hurt.
Maybe it's a jealous acquaintance. If Kagami is really good at something, like she's a star in a particular activity, someone who is jealous of her success might set up a situation where she gets hurt. For instance, they could tamper with her equipment in a way that causes her to have an accident.
In some cases, it could be herself. Maybe she pushed herself too hard in her work or training, and her body or mind couldn't take it anymore. This self - inflicted harm could be a key part of the fanfiction's plot, as she then has to deal with the consequences and learn from her over - zealousness.
It could be an enemy from the underworld. Maybe a powerful demon or a vengeful spirit that has a grudge against her. They might use their dark powers to harm her, like sending a bolt of dark energy her way.