An option for guest display scaling would be useful on high-DPI displays
I use GNOME Boxes on several of my machines and really like it.
I've recently bought a laptop with a high-resolution display (2880x1800). I use it with display scaling in GNOME set to 2x, so essentially it's a 1440x900 in usable area with everything double sized.
GNOME Boxes doesn't seem to have an option to zoom in or scale the guest up - it's always 1 pixel = 1 pixel. This is fine if I'm running a modern guest OS that has its own scaling support, e.g. Windows 10 or a modern Linux distro. I can just give them double the resolution I want, and let them handle their own scaling. But it's a problem when running an older OS from before high-DPI diplays were a thing. Everything is stuck in 1:1 mode and the UI is unusably small.
It would be really nice if GNOME Boxes had a setting for display scale, that you could set per machine and would be remembered the next time you start the same machine. This would make old operating systems far more usable on hosts with high-resolution displays.
Here are screenshots of how Kubuntu 8.04 and Windows 2000 look on my laptop in GNOME Boxes 42.2.