To Vanquish Aku: each PC is the world champion of a different era (anything from ancient Egypt to the 20th century), Samurai Jack style. Each one was kicked into the far future by Aku and remembers Aku arriving in their parents’ generation. They find each other in the Aku-dominated world and team up to finally take down the big bad.
In Samurai Jack, Odin, Ra, and Vishnu appeared as the monks who created his sword, but each PC could vary— e.g.: Sekhmet, Athena, and Freya.
There are numerous eras and historical figures you can use for character ideas: Roland, Joan of Arc, Ashoka, Annie Oakley, Alexander the Great, Sindbad... be sure to pick up the https://www.rejectedprincesses.com/ books for ideas!