A group of humans who are aware of the supernatural world and are on a mission to 'cleanse' it might target Liam. They might think that by kidnapping and getting rid of him, they are making the world a safer place for humans. They could be a secret organization that has been observing Liam for a while and finally decides to make their move.
It could be another werewolf from a different pack. There are often rivalries between packs in the Teen Wolf universe, and someone might see Liam as a way to gain an advantage over his pack.
A vengeful witch could be the kidnapper. Maybe Liam accidentally crossed paths with a witch and unknowingly did something to anger her. Witches in the show have powerful magic and could easily kidnap him and hold him captive in a magically protected place.
Well, it could be that Liam is kidnapped by a rival pack who wants to use him to get information about his own pack. Maybe they think he's a weak link and can be easily manipulated.
Common themes revolve around his insecurities. As a young werewolf, he has a lot of insecurities which are often explored in fanfiction. His place in the pack is another theme. Is he an outcast? Or is he a key member? How he proves himself within the pack is often written about. And the theme of facing danger together with the pack, like fighting against other supernatural threats, is also quite common.
One great 'liam dunbar oc fanfiction' is 'Liam's New Beginning'. It has an interesting original character who helps Liam deal with his werewolf issues in a unique way. The story is well - paced and the interactions between the OC and Liam are very engaging.
Well, I really liked 'Liam Dunbar: The Lost Years'. It's a fanfiction that fills in the gaps of Liam's past, giving more depth to his character. 'Liam in the Parallel World' is also quite fascinating. It imagines Liam in a different world with different challenges. Then there's 'Liam and the Forbidden Friendship' which shows his relationship with an unlikely character in an interesting way.
One common theme is the struggle with his werewolf nature. For example, he might get hurt while trying to control his transformations, especially in difficult situations like during a full moon. Another theme could be him getting injured while protecting his friends or pack members. This shows his loyalty and the risks he's willing to take for those he cares about.
Fanfiction.net is also a good place. Just type in 'liam dunbar oc' in the search bar. You might have to dig a bit deeper compared to AO3, but there are still some hidden gems there. Some authors post their works exclusively on this site.
One common theme is innocence. Since it's about baby Liam, his pure and innocent nature is often highlighted. Another theme is protection. Other characters in the fanfics usually feel very protective of the baby Liam.
It could be Dr. Eggman. He's always after Sonic and would love to get his hands on him to stop Sonic from foiling his evil plans. He might want to study Sonic's speed and use it for his own machines.
It could be Zeref. Since Zeref has an interest in powerful magic users and Natsu is a strong dragon slayer, Zeref might kidnap Natsu to study his magic or use him in some dark plan related to his quest for immortality or power.
It could be an orc warband. Orcs have always been enemies of the elves, and they might see Legolas as a valuable captive either for ransom or to torture. Another possibility is a rogue wizard. Wizards who have turned to the dark side often seek powerful beings to experiment on, and Legolas, with his elven magic and skills, would be an interesting subject.