I and others would love to see live and forecasted air quality data included in the maps. Of most immediate concern is PM 2.5 fine particulate matter ("fine dust"), a very serious and daily health concern in many of the most highly populated areas of the world, and then also climate change-related data.
The data itself seems plentiful and easy enough to gather, for example through aqicn.org.
Here's their data api, free to use.
http://aqicn.org/faq/2015-09-18/map-web-service-real-time-air-quality-tile-api/