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.

http://kaosx.tk/packages/index.php?subdir=kde-next&sortby=name

13 days later

Qt 5.12.0 is now final:
http://blog.qt.io/blog/2018/12/06/qt-5-12-lts-released/
This means that all has moved to the build repo, Qt 5.120 is available there and Fraeworks 5.53.0 is build on top of it, Plasma 5.14.4 rebuild. Any other needed rebuilds have also moved from kde-next, which is now closed again.