Interesting question, as at present RoonOS as part of ROCK running on a NUC does everything I want of it, as a place to run Roon Core to serve my endpoints & zones.
As such I want RoonOS to run as light and fast as possible, with as little resource usage. Additional functionality normally means additional resource usage, so for me “less is definitely more”.
Out of interest I would like a load monitor, with lightweight probes, something like istat with CPU utilisation, load, memory/cache usage and network throughput. I have istat running on my ReadyNAS units, have ported to a RPi and a legacy Linksys NSLU2. I view the statistics on a iOS app. This means the chart rendering is performed off-server, with just the probes running in a thread caching a limited level of data.
On the subject of “something shown on the HDMI (chromecast UI?)”, just get a Chromecast dongle for the display! As to run on RoonOS you would have to envoke GPU processing in the OS, and drive a display - taking away from the headless server focus of RoonOS. Stick to what it is intended as, an embedded server-side headless applicance.
If more is needed, the user can install a general purpose OS and install the Core+Remote product, is my view.