The older version of fcitx is available from slackbuild, but I could not manage to use the script to compile the latest version.
mozc (https://github.com/google/mozc) is the most advanced Japanese IME at the moment. It has dropped its support for ibus recently, and this may pose a big problem for Japanese Linux users in the future. Compiling this beast is also painful but I've managed to do that with the latest release (2.17.2111.102) for ibus (1.4.2). There are some dependencies which I had to get from git and svn (except for protobuf which we have already in our repository - just need to pass "use_libprotobuf=1" for GYP_DEFINES).
Now I've spent almost 2 days before giving up to compile fcitx support for mozc (fcitx-mozc). AUR has some very instructive PKGBUILD.
It would be grateful if anyone could compile the latest fcitx and fcitx-mozc (which is basically mozc + mozc supporting fcitx). I will work on my end as well, but I need also someone to help me out here...

Thanks!