AE Scripts and Mamoworld now offer ExpressionTimeline, a new After Effects script that lets you combine and modify expressions without editing the code itself.

This script makes it easier to:

• make expressions be active only for a certain period of time
• add more than one expression to a property
• fade expressions in and out smoothly
• transition smoothly from one expression to the next one

Here's a short demo; for a full tutorial visit Mamoworld: