History: What can add interest over time to our synthesizer?

What can add interest over time to our synthesizer?

• Panning.
  Panning doesn't inheritantly change things over time, unless...
• Filtering.
  Filtering doesn't inheritantly change things over time, unless...
• Adding distortion.
  Adds interest, but note the "over time" part.
• Modulating using low-frequency oscillators.
  Yes!