Things that are close to my setup would include:
- Weather - Look at developing built-In integrations for local weather stations and sensors, such as EcoWitt and others that offer local comm's
- Lighting - Supporting Philips Hue push notifications in the built-in integration, like in Coco-Hue and Advanced Hue Bridge Integration
- Energy - Start to expand into the energy space, interfacing with services / systems such as Solar and other energy monitoring systems. Of particular interest to me is Solar Edge. Admittedly a cloud service....
- Home Automation Platforms - Develop a built-In integration with Home Assistant, building on the success of integrating with Apple Homekit
Obviously I would also like the UI enhancements many have requested as well, including expanding the capabilities of HE dashboards..... Just still allow CSS and other tweaks that we have so carefully crafted 
If you want to develop a Mitsubishi Thermostat integration, that would also be good,