I just wrote up and issue on introducing a reusable Balloon widget in Cesium, similar to what you find in other projects. This will help making our external data source support much more useful because of existing meta-data in KML and GeoJSON. Obviously we would add CZML support as well. Check it out and let me know if you have any feature requests or any other thoughts in general. I’m not working on this yet, so if someone else has an interest, you are free to give it a shot. Just let us know so we don’t duplicate efforts.