Maybe the rift itself could be seen as an antagonist in a way. It's a force that has taken Owen away from his friends and his normal life. It could be constantly changing and evolving, making it difficult for the Torchwood team to predict its behavior and rescue Owen.
There could also be a rogue element within Torchwood itself that is somehow involved in Owen's being taken by the rift. This person or group might have their own motives, like using Owen's situation to gain power or access to certain Torchwood secrets.
Well, it could be an alien race that has control over the rift and doesn't want Owen to be rescued. They might see him as a threat or an interesting specimen to study.
One possible plot development could be that the team at Torchwood desperately tries to find a way to bring Owen back from wherever the rift has taken him. They might search through old Torchwood files and artifacts for any clues on how to reverse the rift's pull.
One common theme is survival. Given Owen's often - precarious situations in the show, many fanfictions explore how he manages to survive against all odds. For example, in some stories, he has to deal with life - threatening alien encounters alone.
Another interesting one is 'Owen and the Unseen Threat'. In this fanfiction, Owen has to face a new, mysterious threat all by himself while the rest of the team is preoccupied. It shows his resourcefulness and determination, and the author does a great job of creating suspense throughout the story.
I'm not entirely sure as I haven't read it specifically. But generally, in the Torchwood universe, it might deal with the rift and the strange phenomena and battles associated with it. It could involve the Torchwood team facing off against alien threats that come through the rift.
Well, it could be a new inmate who sees Michael as a threat and decides to undermine him. This inmate might spread rumors or false information to get Michael in trouble.
You can try looking for it in large bookstores. They usually have a section for graphic novels, and they might carry the 'Torchwood Rift War Graphic Novel'.
The monsters native to Tartarus could also be antagonists. These could be huge, grotesque creatures that have never seen the outside world. They might attack Percy on sight, seeing him as prey. Some of them could be powerful enough to even give Percy a hard time, especially if his powers are weakened in Tartarus.
It could be some of the nobles in the kingdom who are not happy with the idea of Rapunzel having a baby so soon. They might fear that the baby will disrupt the line of succession or cause political unrest. So they try to undermine Rapunzel and Eugene during her pregnancy, creating all sorts of problems for them.
It could be the Ministry of Magic. They have often been shown to be corrupt and self - serving in the original series. If they try to manipulate Harry again, he might see them as the main antagonists and target them in his revenge.
Maybe the hidden organizations within Konoha that work in the shadows. They could have been involved in some plot against Naruto that led to his decision for revenge. They would be the ones trying to stop Naruto from exposing their secrets and exacting his revenge on the village as a whole.