Here's an idea that I've been thinking about for some time.

Rare events could tie well with randomized sounds; have a bunch of voice clips that get played at random and a few that get picked rarely (like 1/50 times).

As I'm at it, a bit of UI work on those random sounds would be very welcomed. Having to manually browse each time is a bit tedious, so drag n drop and a text field for the sound probability would be absolutely incredible.