Big news: Events will normally use the shape you set in the timeline settings (found at Settings > Event Design > Default Shape), but now you can override the shape for individual events:
To try it, simply add or edit an event and in the Advanced Options section you'll find a Shape option:
Click a shape to select it and use it for the event. Click it again to deselect it and switch back to using the default shape.
This gives you a lot more control over the appearance of the events on your timeline.