Depending on implementation effort, this may be a good candidate for a native feature to ensure performance and forward-compatibility:
Add an option to fade inactive editing panes, so that it’s obvious which pane is focused in multi-pane and multi-window configurations.
Personally, I use the InactivePanes package and find this functionality invaluable for a smooth user experience. (I found InactivePanes while specifically searching for a way to do this, so the need preceded the solution — h/t @FichteFoll.)
InactivePanes has a simple approach which works well, I think: there’s an overlay color with an overlay strength. So if the color is black, a strength of 25% dims all colors in all inactive panes by 25%. A custom color is helpful for consistency with dark/light schemes and theme palettes.
Per https://github.com/SublimeText/InactivePanes/issues/17, InactivePanes may be prohibitive to port to the new .sublime-color-scheme format. Being a rendering tweak, perhaps its functionality might also be better served by native implementation?