Pluggable transports for zero-rated services
Tor increases data usage, and most of the demographics that need Tor the most have very limited data, so please make PTs available for zero-rated services.
For example FreedomPop zero-rates WhatsApp (https://9to5mac.com/2016/08/17/freedompop-whatsapp-sim-free-iphone/) in 30 countries on a free ($0/month) plan.
A WhatsApp plugable transport would therefor allow users in poverty-stricken, massively oppressed countries to have unlimited free access to information through Tor.
I'm not sure if the component should be Orbot or Tor; the WhatsApp example is for mobile but there is also zero-rating for house internet, or someone might want to use a WhatsApp PT on a desktop/laptop/notebook with conventional Tor, tethered to a smartphone for the zero-rated protocol.