Obviously, Quinn is a main character as she is the one who is pregnant. Finn might also be heavily involved as he was Quinn's boyfriend in the show and could play different roles in the fanfiction, like being a supportive partner or facing difficulties in dealing with the pregnancy.
Puck could also be a key character. Since he was involved with Quinn in the original show in relation to her pregnancy situation, in the fanfiction he could be a source of support, or add some drama and conflict to the story depending on how the writer decides to portray him.
Quinn Fabray could be involved too. Given her history of being concerned about her looks and status, a weight gain story for her could be about her learning to love herself regardless of the number on the scale. It could also involve her relationship with the other Glee club members as they support her through the process.
Well, obviously Gaster is the central character as it's about his pregnancy. But often, other characters from the same fictional world might be involved. For instance, characters who are friends or colleagues of Gaster could be there to support or oppose the situation. It might also include characters who have some sort of special connection to Gaster's unborn child, like a potential mentor or an enemy who wants to harm the baby.
Obviously, Bella Swan and Edward Cullen are the main characters. Since it's about Bella being pregnant, they are at the center of the story.
Gale Hawthorne might also be involved. Since he has a complex relationship with Katniss in the original story, his reaction to Katniss's pregnancy could be an important part of the fanfiction. He might have feelings of jealousy or regret, or he could be a supportive friend in a different way.
Kurt Hummel could be another. He is known for breaking gender norms in the show. A male pregnancy plot for him could be a way to further explore his character's journey and how he navigates such an extraordinary situation, as well as how his friends and family react.
Obviously, Daenerys herself is the central character. But often, characters like Tyrion Lannister might be involved. He could be her advisor during her pregnancy, providing his sharp political insights. Jon Snow could also be a part of it, perhaps in a more complex relationship with Daenerys given their previous interactions in the original story. And of course, her loyal followers like Missandei and Grey Worm would likely be by her side, protecting her and the unborn child.
Often, other Decepticons like Starscream might be involved. Starscream, always vying for power, might see Megatron's condition as an opportunity to finally take over. So he could play a major role in the fanfiction, either causing trouble or having a change of heart. And then there could be some human characters who get caught up in the events, perhaps scientists or soldiers who are trying to understand what's going on with Megatron.
Max herself could be a main character in such fanfics. Since she's the central figure in the 'Maximum Ride' series, it's likely that a story about pregnancy would revolve around her.
Obviously, Asuna and Kirito are the main characters. Asuna being pregnant, Kirito would be by her side, protecting her and preparing for the new arrival. Their relationship would be the central focus, showing how they support each other through this new phase.
Alina is also frequently involved. Her connection to the Darkling in the original work makes her a natural addition in many fanfictions. Their complex relationship can be further explored or even rewritten in different ways in fanfiction.