It could be his love interest, like I mentioned before, Lydia. She might reject him because she's too focused on her own goals or perhaps she doesn't see Stiles in that way at the time. Their relationship has always been a bit complex in the show, so this would be a likely scenario in fanfiction.
One possible plot could be that Stiles gets rejected by his long - time crush, Lydia. After the rejection, he might go into a bit of a self - discovery phase. He could start focusing more on his friendships and his own personal growth, realizing that he doesn't need someone else's approval to be happy.
Well, often in these fanfictions, Stiles becomes more confident. His physical appearance might also change, like he could start dressing better or have a new haircut that gives him a 'hot' look. For example, he might start wearing more stylish clothes that show off his personality.
Another character he could interact with is Thor. Thor's otherworldly nature and his powerful abilities would be quite a contrast to Stiles. Maybe Stiles could help Thor understand some earthly concepts while Thor could teach Stiles about Asgardian culture and power.
The fanfiction could be about the adventures Stiles and the puppy have. They could go on hikes together in the forest near his town. During these hikes, they might encounter strange things or other supernatural beings. The puppy could either be scared or curiously investigate. Stiles would have to protect the puppy and also figure out how to deal with whatever they come across.
There might be a jealous friend or acquaintance who feels that Stiles, as a girl, is getting too much attention. This person could then start spreading rumors or causing trouble for Stiles in an attempt to bring her down.
His best friend could discover it. Since they are so close, they might notice something different about Stiles that makes them suspect he's actually a girl.
A kind stranger could come to his aid. Maybe someone who has been through a similar situation in the past. This person could offer practical advice on how to survive while being on the run and pregnant. They might also teach Stiles some skills like cooking healthy meals for the baby's sake.
His relationship with his friends would likely become more strained. They might be afraid of him or he might be afraid of hurting them. For example, his best friend Scott, who is also involved in the supernatural world, might be worried about Stiles losing his humanity.
In this fanfiction, after Stiles gets bitten by a vampire, he might find himself in the middle of a vampire war. The vampire who bit him could belong to one faction, and now the other faction sees Stiles as a threat or a tool. He has to learn about vampire politics and hierarchies quickly. His friends, especially those who know about the supernatural, would try to protect him. Stiles would also have to deal with the physical and emotional changes that come with being a vampire, like the isolation that can come from living a nocturnal life and the fear of hurting those he loves.
Perhaps in this fanfiction, Stiles has disobeyed Peter's orders which were crucial for their safety or the safety of others. Peter, being frustrated and feeling responsible, resorts to spanking as a way to show his extreme displeasure. But this kind of plot would also need to deal with the aftermath, like Stiles' reaction of anger, hurt or maybe even understanding, and how it affects their relationship moving forward.