To add onto the idea, how about allowing the window to be scaled? So, you have a 1600x1200 being blown up to fit properly in a larger space. It would definitely solve the issue of 4K, 5K, and 8K displays having tiny viewports, whilst also allowing the window to be "stretched" to fit screens if the user wishes. It would have an issue of stretching the visuals, but that's to be expected.
For some older games up to dx8 http://dege.freeweb.hu/dgVoodoo2/dgVoodoo2.html might be a solution. It works with dxvk and damm it even has a few low level scaling options including "scanlines". I had some very great experience with games which are else hard to play (example wizardry 8).
Something like ENABLE_DGVOODO2=1 %command% could make the override for the dll's maybe even systemwide so you don't need to copy something into the game itself.
I've been finding some commercial releases of old games bundling dgVoodoo2; I'd be for Steam increasing its usage to more games that would benefit that aren't being bundled with it.
ENABLE_DGVOODO2=1 %command%x1 2018-09
Some games such as Knight of the Old Republic have known issues with fullscreen mode in both Windows, upstream WINE and Proton - for the aforementioned game, the only way to reliably run the game is the set it to run in windowed mode. Without mods, 1600x1200 is the highest resolution.
On a 1440p screen that's pretty ugly looking, for a 4K screen that's awfully small, I'd hate to see it on 5K or 8K...
An option to allow windowed games to be upscaled to fullscreen (by Proton/Steam) would alleviate this and could as an added bonus be used for upscaling games with low resolutions to higher screen resolutions with nice filters like lanczos or spline36 (possibly with a low-impact bilinear option too)
GeDoSaTo is possibly a good point of reference (though it typically is used for supersampling - that wouldn't go astray either, Linux support for VSR on AMD is non existent, not sure for DSR on nVidia - but that might be out of the scope of this request whilst tangibly related)