Ooh, shelve that idea, I just had a better one: what if the players each got a crucial role to play in the story? This might sound obvious, but it's probably not what you're thinking. Like, each player needs to have a crucial, plot-driving role, kind of like my Witch. For example, someone could be the Emperor of Hume, or the Elf Queen's champion, or the Witch's right hand. Each role should be important to the plot, such that the story would not work without them. That way, each player feels important and motivated to write.

This would require a bit more worldbuilding, though.