They could interact in a very confrontational way. Michael Jackson, with his kind heart, would likely be appalled by Muzan's evil deeds. He might use his music as a weapon to try and drive Muzan away or at least make him see the error of his ways.
Muzan was ultimately defeated by the collective efforts of Tanjiro, Nezuko, and the other demon slayers. Their teamwork and perseverance were key to ending Muzan's reign of terror in the manga.
Well, muzan x reader fanfiction often explores a very interesting relationship. In such fanfictions, the writer usually dives deep into the dynamic between muzan, who is a complex and often dark character, and the reader - insert, which can be a very exciting exploration of power, mystery, and sometimes a touch of romance. It allows fans to put themselves in a fictional world with muzan and experience different scenarios.
Transformation is another popular theme. It could be the reader - insert changing muzan, or the other way around. Maybe the reader - insert is transformed into a being like muzan, or muzan starts to change his ways due to the influence of the reader - insert. These themes add depth and intrigue to the muzan x reader fanfiction.
Well, identity is a common theme too. In 'female Muzan fanfic', the exploration of what it means to be Muzan as a female in a world that is mostly dominated by male demons. There's also the theme of survival. How does a female Muzan survive in a world full of threats? Some fanfics might show her using her wits and power to outsmart her enemies and stay on top.
One common theme is the exploration of femininity in a typically male - dominated character. Since Muzan is male in the original, making her female in fanfiction allows for a different take on power and how a female demon might rule.
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.