Android introduced a feature recently in QRP2 where you can force themed Icons to a monochrome scheme based on Material you, even those who don't officially support material you Icons. It allows you to have a cohesive layout with all your icons themed, without any one popping out by having it's own colors. Additionally there is another feature that allows you to theme all icons on the app drawer.
This would be an amazing tool for customization, and easy to implement as the option is baked and the only specail modification needed is for the developer to give us the toggle to enable forced theme icons, and the toggle to enale them in the app drawer if desired.
I've seen it implemented in custom ROMs like Evolution X that are based on AOSP. It's a very simple feature to add, but one that could add so much value.