The latest release of KDE Plasma (5.8) brought a new feature - it can open the desktop menu when the meta (super, Windows...) key is pressed. Like in Windows XP, remember? Alas, it's not yet possible to configure this to do something more useful in the GUI, however weird this seems in a LTS release. After digging in KWin's source code for a while, I found a way to set custom shortcuts in the config files.
First, make sure KWin isn't running. I recommend opening a terminal and running
killall kwin_x11. Now let's say we'd like to open krunner when Meta is pressed
and the desktop menu should be opened with Shift. Edit ~/.config/kwinrc and
add the following lines:
[ModifierOnlyShortuts]
Meta=org.kde.krunner,/App,,display
Shift=org.kde.plasmashell,/PlasmaShell,org.kde.PlasmaShell,activateLauncherMenu
Alt=
Control=
The double comma in the Meta value is not a typo. It means something like that
the called function is not in a namespace, in contrast to the function in the
Shift value. An empty value means no action will be bound to the modifier.
Now, you just need to run KWin again with kwin_x11 --replace and everything
should work. Note that it should be possible to bind any qdbus call to a
modifier key like this. Try using qdbusviewer to explore the possibilites!