Maybe the social system itself could be an antagonist. The bureaucracy and red tape in getting help or assistance for the homeless could be portrayed as something that stands in the way of Brittany and Santana getting back on their feet. There could be long waiting lists for shelters or welfare programs that they have to deal with.
It could be some local bullies in the area where they end up homeless. These bullies might try to steal their few belongings or make their already difficult situation worse by harassing them.
The main antagonists could be the landlords or property owners who are cold - hearted and evict them without mercy, leading to their homelessness.
Perhaps they are both homeless due to a natural disaster that destroyed their homes. They meet in a homeless shelter and form a support group within the shelter. They also try to raise awareness about the plight of the homeless in their community while also trying to improve their own situation.
One common theme is love and acceptance. Their relationship often has to deal with the acceptance from others and themselves. So a lot of fanfics explore how they overcome those challenges.
A group of mischievous animals could also be antagonists. They might be jealous of the attention the baby gets from all the other animals in the neighborhood. So, they start playing pranks on Alvin and Brittany, like stealing the baby's toys or making a mess in their yard. But in the end, they could learn to accept the baby and become friends.
Their relationship would likely become more deep and committed. Having a baby together means they are now a family, and they would have to rely on each other more than ever.
In the fanfiction, perhaps the baby brings out a new side of both Santana and Brittany. Santana could become more protective and gentle, while Brittany might show great motherly instincts. They could also have funny moments, like when the baby first starts to crawl and they are chasing after it all over the place, with lots of laughter and chaos in their home.
There's a fanfic called 'Love in the Choir Room'. It delves into how Brittany and Santana's relationship might have evolved during their time in the choir. It has some really interesting plot twists and great character interactions. It shows their love story in a very unique and engaging way.
A jealous classmate could be a main antagonist. This person might be envious of Naruto's popularity and his ability to make friends easily. They could try to spread rumors or sabotage Naruto's projects.
The Akatsuki could still be a major antagonist. Even if Naruto is emperor, they might oppose his rule as it goes against their own goals of chaos and obtaining the tailed beasts.
One possible antagonist could be a jealous noble from Naruto's own kingdom who feels that Naruto doesn't deserve the prince title. This person might try to undermine Naruto at every turn, spreading false rumors about him or trying to sabotage his diplomatic efforts.
Another possible antagonist could be a fellow inmate who has a personal vendetta against Naruto. Maybe this inmate was once on the wrong side of Naruto's actions outside the prison and now sees this as an opportunity for revenge. He could try to undermine Naruto at every turn, from spreading false rumors about him to physically attacking him when the guards aren't looking.