There's even a "custom" waybar module that supports various triggers/actions defined via JSON. A similar module could be written using mruby that calls DSL methods defined in a Ruby config file.
https://github.com/Alexays/Waybar/blob/master/src/modules/custom.cpp
https://man.archlinux.org/man/waybar-custom.5.en
Any desktop app, widget, or service that embeds Lua or JavaScript for it's configuration or plugin system could embed mruby; either by adding an optional build flag, creating an mruby specific fork of the project, or via a plugin that hooks mruby in. Lots of potential to integrate Ruby into the desktop.
https://github.com/mruby/mruby#readme
https://github.com/mrubyc/mrubyc#readme
Another free project idea: write a generic waybar module that embeds mruby so you could write waybar modules entirely in Ruby.
Using Lua as the configuration syntax or plugin system for a Window Manager isn't a new idea. AwesomeWM , a X11 Window Manager, uses Lua as it's configuration syntax. Now imagine being able to customize your fancy wayland tiling WM or add additional functionality with simple Ruby scripts running under mruby.
Weston (the Wayland example/reference compositor) added a custom Lua based shell that you can configure/extend with Lua. I wonder how much work it would take to build a mruby Weston shell? I would prefer a Ruby DSL over Lua.
Ruby's humble Rake could use a slick website, similar to https://bundler.io.
postmodern
Jeff Kreeftmeijer