One great 'Usagi Drop' pregnancy - related fanfic might be the one that focuses on how the characters adjust to the new addition in their family. It could explore the emotional journey of the main characters as they prepare for the baby.
I'm not sure which ones are the most popular specifically, but you can start by looking on fanfic websites. Check the tags related to 'Usagi Drop' and 'pregnancy'. There might be some fanfics with high ratings or a large number of reviews that could be great.
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.
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.
Tuxedo Mask could also be in the harem. He has a long - standing relationship with Usagi, and in a harem scenario, his feelings for her could be even more intense, competing with others for her love.
Well, 'OP' in the context of this 'Fury Tries to Kidnap OP Harry Potter Fanfiction' could be the main character that Fury is targeting because of their special powers. Maybe it's a new character created by the author with unique magical abilities that Fury, for some reason, wants to get his hands on. It could be someone with a combination of different magical skills from the original Harry Potter universe, like being able to perform extremely powerful spells without a wand or having the ability to control all types of magical creatures.
Another reason could be an accident. In some fanfictions, the setting might be more of a daily - life scenario with a twist. Perhaps Usagi is distracted while walking and trips over something, resulting in an injury. This can add a touch of realism to the otherwise fantastical world she lives in.