I changed the deps, this is the diff:

-depends=('qt5-graphicaleffects' 'libxkbfile' 'libusb-compat' 'kinfocenter'
- 'systemsettings' 'ksysguard' 'powerdevil' 'oxygen' 'kmenuedit'
- 'polkit-kde-frameworks')
+depends=('qt5-graphicaleffects' 'libxkbfile' 'libusb-compat'
+ 'powerdevil' 'kwin' 'libcanberra' 'polkit-kde-frameworks')

BTW, why adding oxygen, kmenuedit, systemsettings as deps here? Just to be sure the user installed them or they are explicitly stated somewhere?
I really don't get the difference between plasma-workspace and plasma-desktop. I'm in the same situation with breeze and milou, I removed them from the plasma-workspace's deps, and add them to the plasma-next group.

See [1] for the "official" dependencies. plasma-workspace is supposed to contain the common bits for all form factors (desktop, active and mediacenter for now), plasma-desktop is the desktop specific stuff.
The reason why they are needed:
- oxygen: contains the default window decoration, if it's not installed kwin will use aurorae, which is buggy and causes crashes (see the Errata). It should really be a dep of kwin, but it would create a circular dependency so it's not possible. In 5.2 there will be a native breeze windeco so it won't be needed anymore.
- kmenuedit: needed for right-click on the kickoff or kicker button -> edit applications
- systemsettings: it's in the default favorites, needed for basic configuration
- milou: it provides the GUI for krunner, which is an important part of the workspace (they should really update the description upstream, it has nothing to do with baloo anymore)
- breeze: contains the default wallpaper and cursor theme for p-w

[1] http://quickgit.kde.org/?p=kde-build-metadata.git&a=blob&h=0e6fae2d6cdb1bbc88f126ff9f5e93b9bd810b10&hb=f0737b9914b5e40577304a93af468053473f48a9&f=dependency-data-kf5-qt5

Thank you very much for your explanation and work!