Technically it’s pretty straightforward to add LCDs and buttons to OSPi. It will just need to take a few extra GPIO pins. However, the usability design, particularly how to stack everything together and still keep the unit compact, is more tricky. I can’t think of a really elegant design without involving three layers (OSPi, RPi, and display/buttons), and this number of layers would also make the enclosure look really ugly.
Ultimately the plan should be to integrate RPi (or any embedded Linux chip) together with the OSPi board. This way the product will look exactly like the current mcu-based OpenSprinkler but runs embedded Linux internally. I am investigating this option but still pretty far from having the first functioning prototype.