Friend of mine looked into smart sprinkler systems, and said he would buy any of these if they could connect up to his Mozilla Things Gateway. What makes a commercial device easier or harder to create an add-on adapter for? Any thoughts on which of the following would be most straightforward to develop an add-on for? Any volunteers? 
My experience with sprinkler systems is that there are zones, and times, and you typically program them for time of day to start, how long for each zone, and which days of the week. For starters, the existing rule system could program a zone to be on at a certain time, and another rule for off at a certain time. (Eventually rules will have duration which will help too.)
If we added a “day of week” widget (like the clock) as an input, then an “and” rule could cover time of on or off + day of week resulting in sprinkler zone on and off states.
8-Zone B-hyve Indoor Timer
$80
https://www.homedepot.com/p/Orbit-8-Zone-B-hyve-Indoor-Timer-57925/303722145
B-hyve 6-Zone Indoor/Outdoor Smart Sprinkler Controller, Works with Amazon Alexa
$100
https://www.homedepot.com/p/Orbit-B-hyve-6-Zone-Indoor-Outdoor-Smart-Sprinkler-Controller-Works-with-Amazon-Alexa-57946/206638404
Gro 7 Zone Controller - Smart Sprinker - 7 Zone Sprinkler / Irrigation Controller - Wifi Enabled, Remote Access
$100
https://www.homedepot.com/p/Scotts-Gro-7-Zone-Controller-Smart-Sprinker-7-Zone-Sprinkler-Irrigation-Controller-Wifi-Enabled-Remote-Access-70001-1/302953648
Smart Sprinkler Controller, Wi-Fi, 8-Zone 2nd Generation
$180
https://www.homedepot.com/p/Rachio-Smart-Sprinkler-Controller-Wi-Fi-8-Zone-2nd-Generation-8ZULW-B/207015682
…and then there’s this 
OpenSprinkler Pi (Sprinkler Board for Raspberry Pi)
$80
https://www.amazon.com/OpenSprinkler-Pi-Sprinkler-Board-Raspberry/dp/B01MRRXZP0
https://opensprinkler.com/
I can create a github issue once we chat a bit about the scope and which product(s) to target first.