For me Roon OS 2.0
Implement a frame buffer for HDMI on Nucleus and NUC. For now just a Roon logo in 720p with IP details would be better than the text screen which just doesn’t look very “pro” for a £1000+ device. Even my Sonos Beam outputs a pretty picture. In time it could be used for Now Playing or status page or whatever. Personally I vote for Now Playing.
HDMI-CEC volume control would be very nice but I appreciate only a few hardware variants support it.
Sandboxed extensions installable on the Nucleus/ROCK without having to use another computer to run them. Presumably done in such a way (virtual machine - Docker etc) that a crashing extension won’t bring down the core OS.