Fullscreen is an improvement I would like to add soon. It wasn't working well during development because I actually run it with two video players - with the hidden one loading the next video so that the cutover is quick.
The videos are shown at their highest-possible resolution in the window now, though, so at least we aren't missing out on any quality (though it would be great if Instagram allowed users to upload higher resolution source videos).
One thing that I would point out about the script you intend to write is that it requires an awful lot of maintenance (when sites change layout) and is frequently not very reusable. One solution that I tried is Mozenda (http://www.mozenda.com). The have all the stuff you're looking for (i.e. visual, browser-based tool, writing to XML) but also have error handling and notifications, so that if an agent breaks you'll know and be able to fix things inside the visual tool.
