One idea could be a story where Yuji and Nobara go on a special mission together in an unknown location filled with new types of curses. They have to rely on each other's strengths more than ever.
One idea could be exploring the backstories of the side characters more in - depth. For example, you could write about how Maki became so strong - willed. Another idea is to create an alternate universe where the curses and sorcerers have different power dynamics. Maybe the curses are the ones in control this time. And a third idea would be to focus on the relationships between characters outside of the main plot, like how Yuji and Todo's friendship could develop further in different scenarios.
One interesting idea could be the characters going back in time to prevent certain tragedies. For example, they might try to stop the deaths of some important characters. This could change the whole course of the story and create new relationships and plot twists.