The kde-next repo is re-opened for the next major update of Qt. 5.12.0RC is now up, full rebuilds of Frameworks & Plasma are also included. A few apps are already rebuild, expect there will be more broken, thus needing a rebuild on new Qt.
Report any issues you may find for Qt 5.12 here.
As always, make sure kde-next is the very first repo in your pacman.conf and DO NOT use kde-next if you do not know how to add repo's. This is not for regular use, expect issues & breakage.