I see one of two options. You fully open source all of your code, especially on the controller or you create a plugin mechanism possibly with its own app ecosystem.
As a far fetched example, I also have a solar rooftop system which could be a basis for understanding how much solar radiation my house is getting which could influence the watering time of the rachio. Or I could build the mechanism to add the rachio into a larger home automation framework. Or because I am really interested in having my two separate rachio controllers operate as if they were a single unit maybe I would be willing to do the development work and contribute it to the open source community. Its a model that has worked quite well in the software industry even through it can be counter intuitive.
Because it is either open source or an app ecosystem Rachio would benefit by having other technical individuals contributing to the capability of the system. Because you maintain control of the open source you can ensure the quality of the changes that actually make it out into a release.