"Lifelike responses" — nice.

I like that "action potential" idea – I was trying to think of a good implementation for "do something twice a month per user" — decaying counter is a nice idea.

I'm with you on willy nilly making parts of your UI random — my use case is for changing up the text in greetings, general play with copy, and once in a while showing something like a tip or a "welcome back" message. Basically, small touches that convey a small sense of variety (vs. RANDOMLY DONT SHOW THE NAV!)