There is a fanfic titled 'The Bond of Chibiusa and Her Sister'. In this, it delves deep into how they support each other through different challenges in their magical world. It's a really engaging read as it also shows their growth as characters.
Usagi's friends would be extremely worried. They would immediately start looking for her. They might split up to cover more ground and search different areas where Demando could be hiding her. They would also use their powers to try to sense Usagi's location. For example, if one of them has a power related to sensing emotions, they could try to detect Usagi's fear or distress.
One popular theme could be the exploration of their relationship in different magical scenarios. For example, they might be on a quest together in a new, uncharted part of the magical world, which tests and deepens their bond. Another theme could be the power of their combined magic. Since they both have unique magical abilities, fanfiction often delves into how they use these powers to overcome challenges. Also, the theme of self - discovery within their relationship is quite common. As they interact more, they learn more about themselves and their place in the magical realm.
One popular 'chibiusa older sister fanfic' could be 'Chibiusa's Big Sister Adventures'. It often shows Chibiusa in different situations with her older sister figure, exploring their relationship and the challenges they face together.
Perhaps a story where Chibiusa has a dream on her seventh birthday. In the dream, she travels to a different time and meets her future self. Her future self gives her some advice and wisdom that she can use as she grows up.
Well, 'Journey of Hearts' is also a great one. It begins with a chance encounter between Gohan and Chibiusa. The story then delves into how they gradually open up to each other, sharing their hopes, fears, and dreams. It's a really heartwarming fanfic that shows their relationship developing in a very natural way.
Archive of Our Own (AO3) is another great place. It has a very organized tagging system. So, you can search for 'gohan x chibiusa' and it will show you all the relevant fanfics. There are also filters you can use to sort by things like word count or rating.
Love and sacrifice is a common theme. Mamoru would sacrifice his time and energy to take care of Usagi. Another theme could be the power of friendship. Their friends would likely come to their aid during this difficult time. Also, the theme of hope. Despite Usagi being sick, there is always hope for her to recover and have a healthy baby.
The plot could involve Usagi's sickness being related to some sort of magic or enemy plot. Since she's pregnant, her powers might be affected. Mamoru, being the loving partner, searches for a cure while also protecting Usagi. There could be a lot of emotional turmoil as Usagi is worried about the baby and Mamoru is torn between finding a solution and being by her side all the time. Eventually, with the help of their friends, they figure out what's wrong and Usagi recovers, ready to give birth to a healthy baby.