First, a common main idea within myths tends to be the supremacy of the pharoah. Ancient Egyptians readily believed that the pharoahs were gods, so in every story, a pharoah would be remembered and "idolized". Another common idea would have, similarly, pertained the the mummies of the pharoahs. Stories of them reaching the underworld or stories of them being upset with their burial would have been common during the time period.
There would have been two common settings. The first would have been within Egypt itself. In Ancient Egypt, not many people had traveled extremely long distances. Thus, Egypt would have been home to their stories. A second setting (and theme) would have been creation. All societies at this time were obsessed with how creation began, and the Egyptians are no exception to this.
Finally, the main lesson would have probably revolved around remembering the supremacy of the Pharoah.