Well, Inosuke's origin story is quite interesting. He was found in the mountains as a baby and was raised by a family of boars. This led to his rather unorthodox and savage behavior. He didn't have any proper human upbringing which is why he often acts on his instincts. For example, he is very aggressive in battles and doesn't really follow the normal social norms that other characters do. His origin also influenced his unique fighting style which is very much based on brute force and his animal - like agility.
One common theme could be adventure. Since Inosuke is a rather wild and adventurous character, fanfics might involve him and AOI going on some sort of exciting journey together, perhaps exploring a new and dangerous area.
Protection is a common theme. Tanjiro is always looking out for Nezuko, whether it's from other demons or humans who fear her. Another theme is loyalty. Nezuko's loyalty to Tanjiro is unshakable and this is often explored in fanfics.
One common theme is the growth of their relationship. It often starts from a simple acquaintance and develops into a deep emotional bond. For example, they might start as comrades in the fight against demons and gradually fall in love.
Love and friendship are very common themes. The fanfics often explore how the reader and Tanjiro develop a deep bond of love or friendship. Another common theme is adventure. Since Tanjiro is on a journey in the original story, many fanfics incorporate the reader into his adventures.
One common theme is the exploration of their different personalities coming together. Tanjiro is very kind - hearted and Shinobu has a more complex, somewhat mysterious nature. Fanfics often show how they bridge that gap.
I'm not sure if there is such specific 'muzan x tanjiro mpreg fanfiction' available. Fanfiction can cover a wide range of themes and pairings, but this one might be a rather niche or even against the general tone of the original work in some ways. You could try searching on some large fanfiction platforms, but don't expect to find it easily as it may not be very common or might be against the platform's rules if it contains inappropriate content.
One common theme is friendship. They often start as comrades in the Demon Slayer Corps and develop a deep friendship. For example, they support each other in battles.
Yes, there are quite a few. Some focus on an alternate universe where they have to team up for some strange reason. For example, in one fanfic, they have to face a new common enemy that is even more powerful than Muzan. Tanjiro has to put aside his hatred for a while to deal with this new threat.