I'd love to see the amount of carbon dioxide emissions prevented from commuting by bicycle, walking, etc. It could be displayed on your activity just like calories burned (but only when "commute" is chosen). Not only could this encourage more people to choose active transportation, but I read on your Commutes on Strava page that "we hope to make the commuting experience on Strava more rewarding and exciting in the future." A win-win!
CO2 Emissions displayed on NH CommuteSmart platform