It could be a mischievous Pokemon. Maybe a Ditto that has gone a bit wild and accidentally turned into a Pokeball and then somehow 'captured' Ash when he touched it.
Well, it could be that Ash gets caught in the Pokeball by accident. Once inside, he might start to understand the feelings of Pokemon when they are captured more deeply. He could try to figure out a way to escape, and in the process, learn some secrets about how Pokeballs work that he never knew before.
In a fanfiction scenario, Ash might be in a situation where he has to use a unique Pokeball to catch Lugia. The Pokeball could have some sort of special power, like being able to withstand Lugia's powerful attacks. Ash would have to figure out how to use this Pokeball effectively while also respecting Lugia's might and the bond that Lugia has with the sea.
Imagine a fanfiction where Ash is exploring an uncharted island. On this island, he finds a Pokeball half - buried in the sand. The Pokeball seems to be made of an unknown material. As he picks it up, a strong gust of wind blows, and he hears a faint cry that sounds like Lugia. He starts to study the Pokeball and realizes that it has a hidden power. This power is related to Lugia's origin. Ash decides to find Lugia using the clues from the Pokeball. As he gets closer to Lugia, he has to face his own doubts and fears. He also has to deal with other Pokemon and trainers who are also interested in the powerful combination of the Pokeball and Lugia.
There's also a possibility that it's an accident. In the chaos of a battle or a tense situation, a stray bullet from either side could hit Katniss, even if no one actually intended to harm her directly.
Another possibility is that it's a rogue Shadowhunter. There are always those in the community who don't agree with the way things are going. If Clary's actions or ideas are seen as too radical, this rogue Shadowhunter might try to stop her by injuring her. This could also lead to a story about Clary trying to understand the motives of this attacker and potentially reform them.
It could be a Sith or a Sith - inspired enemy. They are often the ones causing trouble for the Jedi, and shooting Ahsoka would be a way to strike at the Jedi Order.
It could be a rival assassin. They might see Kassandra as a threat to their own reputation or contracts.
It could be a new, inexperienced doctor. Maybe they make a mistake during a surgery that Meredith is involved in, accidentally causing her harm. For example, they might misread a vital sign or use the wrong instrument.
There's a possibility that it's an old enemy who has somehow regained power or found a new way to attack. For example, Frieza's family or the Red Ribbon Army if they had some kind of resurgence. They might target Vegeta and Bulma out of revenge or to gain access to Capsule Corp's technology which Bulma controls.
A new and powerful rogue ninja could be responsible. This ninja might have a vendetta against Konoha and see Naruto as a symbol of the village, so they target him and use their strong and devious techniques to injure him badly.