This is a follow-up to "Things to note when design a minimal CLI utility", but this time it's about window manager.
Configuration
I have covered this in detail on my previous blog, it just also applies to WM.
Widgets
Widgets like bar, menu don't need and shouldn't be built-in to the windows manager. Because there is already tools out there that handle these widgets far better (EWW, Conky, bars, menus). The exception is if the WM has a whole widget's system like AwesomeWM that can be used to config everything from wallpaper to window decorations.
Wallpaper
WM should not go out of it's way to handle wallpaper, leave it to the wallpaper setters.
Hotkeys
The same go for hotkey, hotkey daemons exist! (SXHKD, Kanata, KMonad). The only exception are to config mouse buttons (with mod keys) to focus, move and resize windows.