I think there should be a command which overlays images on the screen. It can be used to show manuals, maps, or notes while gaming. It could also be used if you're an artist and you need to constantly keep referencing pictures.
I made an experiment in a plugin a while ago, and it worked reasonably well for windowed and/or borderless fullscreen applications.
The biggest issue, which i think stands true even if it was added as a native VA feature, is applying the overlay with "true" fullscreen app: this I'm pretty sure requires hooking directly into Direct3D/OpenGL DLLs, which is not trivial to do, and, worse, always put you at risk of being flagged by Steam VAC/other anti-cheating modules as malicious code