What's the biggest missed opportunity in science fiction history?S

Science fiction's past is full of possibilities that never quite materialized. Movies that didn't get made. Crossovers that didn't happen — like the Doctor Who/Star Trek: Enterprise crossover that was talked about. But what's your most tantalizing "might have been" from science fiction history?

Please include a picture or some kind of audiovisual indicator to help us appreciate the lost glory of your alternate-history event. And for good measure, please throw in some details. Thanks!

TARDIS image via SpaceEngine.org.