Having played around a little with the current timeline implementation, I think we need these three improvements:
As new features are switched on for the first time in a timeline change, they should use the selected style. Then they should go back to regular style when the timeline moves on, if they stay visible.
For each timeline change, the map should automatically be zoomed to show all the currently-visible items.
SVGs should be used for the button captions. Unicode triangles are unreliable on Macs.
Having played around a little with the current timeline implementation, I think we need these three improvements: