I reckon fanfic for these would be great - and a great incentive to encourage people to read these works.

I was surprised that ALL book fandoms were still below 500 works. At this rate, I figure we can nominate Rosemary Sutcliff - works as an exchange category and write straight up crossovers between/among Sutcliff books. (Which I think might shine a light on lesser represented works - one more popular plus one less popular equals crossover? Maybe time travel or some other trope!)