The Origin of Demons and Muzan's Origins:

One theory suggests that there is a deeper connection between Muzan Kibutsuji, the main antagonist of the series, and the demons. Some fans speculate that Muzan may not be the creator of demons himself but rather turned into one through an unexplained power. It is presumed that there is a higher power or an ancient power source that gave rise to demons, and Muzan somehow manipulated or usurped this power. This theory could lead to intriguing revelations about the origins of demons.

Tanjiro's Future Development and the Mystery of His Family:

Another aspect of the series that intrigues many fans is the mystery surrounding Tanjiro's family and its potential influence on his abilities and destiny. Some theories suggest that there is more to Tanjiro's powers than has been revealed so far. This could establish a connection to his family and lead to a discovery that makes Tanjiro an even more powerful demon slayer. This could also hold deeper significance for the overall concept of the series.

The Fate of the Pillars and the "Breath" Techniques:

The various breathing techniques used by the Pillars of the Demon Slayer Corps are a central element of "Demon Slayer." There is speculation that there might be an original breathing technique or a common source for all the breathing techniques utilized by the different Pillars. This theory implies that in the past, there was a unified teaching or a legendary figure who laid the foundation for the various breathing techniques. Revelations about this could not only deepen the history of the Pillars but also reveal a greater threat or a common mission for all demon slayers.