Inosuke survives throughout the manga. The story doesn't have his death as part of the plot. His character remains active and plays a significant role until the end.
In a 'kokushibo meets tanjiro fanfiction', one possibility is that Kokushibo senses something different about Tanjiro right away. Maybe it's the fact that Tanjiro has a unique scent or an energy that Kokushibo has never encountered before. Tanjiro, being Tanjiro, would be ready to face Kokushibo head - on, but also be open to communication if there's a chance. The story could explore their different views on the world of demons and slayers, and how they might come to understand each other better through their encounter.
First, you need to understand Inosuke's character really well. Know his strengths like his amazing physical abilities and his weaknesses like his lack of social graces. Then, choose the world you want to cross him over with. For example, if it's a fantasy world, figure out how his abilities would work there.
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.
Inosuke doesn't become a Hashira in the manga. The story progresses with him having other significant experiences and achievements instead of achieving that specific position.
Inosuke doesn't die. The author keeps him alive to continue his adventures and growth. His character's resilience and fighting skills enable him to avoid death.