The TVTropes Story Generator gives you a bunch of elements you can use as inspiration to build your tropey tale.
Absorb the info, become one with the prompts, and then spawn something ridiculous. Or brilliant. Or ridiculously brilliant. It's all welcome here. Curious to see what else the Story Generator can come up with? Hit New Batch and see what happens!
And don't forget to checkout the official TV Tropes Pitch Generator.