A possible fanfiction could focus on a love triangle between Kurumu, Tsukune and another character. Kurumu is deeply in love with Tsukune, but Tsukune is conflicted as he has feelings for Kurumu but also has responsibilities towards others. This would lead to a lot of emotional drama and interesting character interactions.
One great Tsukune and Kurumu fanfiction could be a story where they go on a magical adventure together outside of the school. They might face some new and powerful enemies that test their relationship and skills.
A common theme is the development of their relationship. Given their different natures in the Rosario + Vampire world, how they come to understand and care for each other is often explored. For example, Tsukune being a human and Kurumu being a succubus.
One great fanfiction could be one where Kurumu and Tsukune go on a secret adventure together in the Rosario Vampire world. It might involve them facing a new type of monster that no one has ever seen before. This would test their relationship and how well they can work together.
One of the themes that frequently pops up is growth. As they fall in love, both Tsukune and Kurumu grow as characters. Tsukune becomes more confident in dealing with the yokai world, and Kurumu learns to be less impulsive in her actions. And of course, the theme of true love against all odds is also very common, with them facing various obstacles but still staying together.
Another option is to check out some anime - themed forums. The fans there are usually quite passionate about the series and might have their own collections or recommendations of Tsukune and Kurumu fanfic. You could ask in a thread specifically dedicated to Rosario + Vampire fan works.
One great tsukune and kurumu fanfiction could be 'Tsukune's Hidden Power'. In this story, Tsukune discovers a new power within himself that Kurumu helps him to control. It's a really exciting read as it shows their relationship growing stronger through this shared experience.
You can try fanfiction.net. It has a large collection of fanfictions from various fandoms, including ones about Tsukune and Kurumu. Just use the search bar and type in their names, and you should be able to find a good number of stories.
Ginei would be a bit more reserved in his reaction. He might offer some practical advice from his own experiences or knowledge. He'd also be there to protect them if any threats came their way during the pregnancy, as he values their friendship.
A third plot could be about how their relationship changes during the pregnancy. Tsukune might become more protective of Kurumu, and Kurumu might discover a new side to Tsukune. They could also have some cute moments like Tsukune talking to the baby bump, and Kurumu teasing him about it.