They would likely become more protective of each other. Kurama would be constantly on guard to keep Botan and the unborn child safe. Their relationship would deepen as they share the new experience of impending parenthood. For example, they might have more tender and private moments together, away from their usual adventures.
Well, Kurama might train Naruto by teaching him how to better control and access chakra. Since Kurama has a vast amount of chakra, it could show Naruto unique ways to harness and use it. For example, it could guide Naruto in creating more powerful and complex jutsu by using their combined chakra in a more efficient way.
In fanfiction, the fact that they come from different parts of the Yu Yu Hakusho world adds to the allure. Botan is a guide to the afterlife and Hiei is a demon. Their different backgrounds and the potential for them to bridge the gap between their worlds can create complex and exciting plots. For example, a story could explore how they deal with the cultural differences between the spirit world and the demon world while falling in love. There could be conflicts arising from their respective duties and loyalties, which can be delved into in great detail in fanfiction.
You can try searching on fanfiction websites like Archive of Our Own (AO3) or FanFiction.net. These platforms have a wide range of fan - created works, and you might be able to find some great Botan and Hiei love fanfiction there.
It could also be due to a misunderstanding. Suppose Naruto accidentally does something that Kurama misinterprets as a threat to its own existence. For example, if Naruto tries a new jutsu that Kurama wrongly perceives as an attempt to suppress it further.
Yes, unfortunately, Kurama passed away in the manga. This occurrence brought about a lot of changes and emotions for both the characters and the readers.