Let's fantasize a bit.
Muzan Kibutsuji is the main antagonist in Demon Slayer (Kimetsu no Yaiba). Muzan has got a very distinctive appearance, he is a tall man with unnaturally pale skin. This demon possesses immense powers and is known for his cruelty and brutality. The character has always been extremely frightening, yet mysterious and appealing, and that's what makes him so interesting to talk about.
Muzan has the ability to change his forms. Along with his main form, he was portrayed as an elegant woman and a young boy. Some fans say that it would be better if the demon's main form was female, because it would increase the impact of his cruelty: women don't usually tend to be depicted as viciously sadistic villains due to their nature.
A number of fans complain about the lack of details about Muzan provided by the series. They would like to see more of him living undercover, using other forms and pretending to be a regular person. Seeing him more embedded into the society would also do the trick. What if the demon was a secret puppeteer of the government? Fans crave for more of his interaction with humans and wonder what his behavior would be like with them.
Another wish is for more clarity about Muzan's plans: what is he going to do after he's achieved the sun immunity? The demon's overall goal also seems quite vague to some viewers, and they can't help wishing it was fixed and are eager to learn more about this fascinating character.
Muzan's strategic skills are also a topic of discussion among the fans. They especially emphasize his relationships with the Twelve Kizuki (the strongest demons there are and the closest ones to the demon king himself). After slayers defeat some of the Upper Moons, Muzan just sends more of them (and not the strongest ones) to the village full of demon slayers. Fans also believe that Kibutsuji should be more careful with the choice for the Kizuki, and give the selection of the ones to turn into demons a bit more thought.
Muzan's organization should be his main focus because these demons are exactly the ones who help him pursue his mission and defeat his enemies.
This is also a reason why fans suggest it would be curious to see Muzan's vengeful side. What if after learning about the death of some Kizuki members, he would unleash his wrath on the entire demon slayer village?
These are all just thoughts and suggestions, of course, and for now we can only wait for the story of Demon Slayer to come back to our screens, and hope that we'll see more of Muzan and his wicked nature.