Allow users to scroll past the end of the timeline - vertically. Right now you can only scroll as far as the last layer but if a timeline window is huge its a pain to only look at the bottom of the timeline to adjust your last layers.
Also, allow users to scroll past the beginning and end of the timeline - horizontally. Use negative timecode or different colors to indicate the beginning and end. Especially with character animation, it would be immensely helpful to slide layers past the beginning of the timeline, say 00:00:00:00, but still be able to adjust those keyframes.