Another possibility is that there is a soul who blames Marcus and Dom for something that happened in their past lives. This vengeful soul could be the antagonist, seeking revenge on them in the afterlife and trying to make their existence there a living hell, so to speak.
It could be a malevolent spirit who is jealous of their friendship. This spirit might try to drive them apart in the afterlife by causing misunderstandings between them.
Maybe there is a powerful entity in the afterlife that doesn't want Marcus and Dom to succeed in their new - found purpose. This entity could be an antagonist as it constantly creates obstacles for them, whether it's physical barriers or mental challenges.
Well, it could be that they meet and reflect on their past lives. Maybe they realize the mistakes they made while alive and try to find a way to make amends in the afterlife.
Marcus and Dom could be two friends who had a very eventful relationship during their lives. Maybe they were soldiers together, or they grew up in the same neighborhood and had many adventures.
One possible plot could be that Marcus and Dom meet in an afterlife realm. Maybe they have unresolved issues from their lives and must face them together. They could relive certain memories, both good and bad, and through that process, find a sense of peace or redemption.
It could be a dark wizard who is trying to prevent Lily and James from meeting Harry. This wizard might be a follower of Voldemort who is still bitter about the downfall of their master and wants to disrupt this family reunion to cause chaos in the wizarding world.
It could be Dr. Eggman. Maybe he has sent some of his robots to the area where Sonic and Tails first meet. The robots are causing chaos and Sonic and Tails have to team up to stop them, which is how they start to bond.
It could be a bully at their school. This bully might be a bigger animal, like a buffalo or a rhino, who picks on Nick because he's a fox. The bully might also tease Judy for being a small bunny with big dreams. Nick and Judy would have to team up to stand up to this bully, which would strengthen their bond.
Another possible antagonist could be a powerful vampire hunter. He sees Damon as a threat and Elena as a potential target or a way to get to Damon. When Damon and Elena meet, this hunter is already watching them. He plans to use their new relationship against them, making their first meeting the start of a lot of trouble for both of them.
Maybe a time - traveling villain from a different dimension who sees Lewis' sister as a threat to his plans of ruling all timelines. He constantly tries to thwart her efforts to make the future a better place. For instance, he might try to disrupt her attempts to introduce new technologies to the Robinson family that could benefit everyone. He would set up various traps and obstacles for her throughout her adventures in different time periods.
A hunter could be the main antagonist. Hunters are often seen as a threat in stories involving wild or unusual creatures. They might be after the werewolf for its pelt or just because they see it as a dangerous beast.
It could be developed by exploring their past. Maybe they start to talk about their childhood memories that they haven't shared before. This kind of deep conversation can make them understand each other better and bring them closer. Another way is through facing a common enemy. They have to protect each other and in the process, their feelings for each other grow stronger. They learn to trust each other more and become more than just friends or partners, but truly soulmates.