really mad proposal: 
add a PDF viewer to read PDF of music sheets on a monitor connected by HDMI
I use SUMATRApdf that I believe is based on open source PDF reader
The ‘monitor connected by hdmi as a second screen’ came up in zynthian club. It would be fun but there are too many configurations. Someone using a pi3 self-build would likely be using the hdmi as the main screen for zynthian-ui where would the ‘second screen’ be displayed?