This is what we currently have as extra package dependencies solely due to the kf5-messagelib library (on fedora 25):
grantlee-qt5.x86_64 5.1.0-2.fc25
kdepim-apps-libs.x86_64 16.12.2-1.fc25
kf5-akonadi-contacts.x86_64 16.12.2-1.fc25
kf5-akonadi-mime.x86_64 16.12.2-1.fc25
kf5-akonadi-search.x86_64 16.12.2-1.fc25
kf5-akonadi-server.x86_64 16.12.2-1.fc25
kf5-grantleetheme.x86_64 16.12.2-1.fc25
kf5-kbookmarks.x86_64 5.31.0-1.fc25
kf5-kcalendarcore.x86_64 16.12.2-1.fc25
kf5-kded.x86_64 5.31.0-1.fc25
kf5-kdelibs4support.x86_64 5.31.0-1.fc25
kf5-kdelibs4support-libs.x86_64 5.31.0-1.fc25
kf5-kemoticons.x86_64 5.31.0-1.fc25
kf5-kidentitymanagement.x86_64 16.12.2-1.fc25
kf5-kimap.x86_64 16.12.2-1.fc25
kf5-kio-file-widgets.x86_64 5.27.0-1.fc25
kf5-kirigami.x86_64 1.1.0-3.fc25
kf5-kitemmodels.x86_64 5.31.0-1.fc25
kf5-kldap.x86_64 16.12.2-1.fc25
kf5-kmailtransport.x86_64 16.12.2-1.fc25
kf5-kmbox.x86_64 16.12.2-1.fc25
kf5-knewstuff.x86_64 5.31.0-1.fc25
kf5-kparts.x86_64 5.31.0-1.fc25
kf5-kpimtextedit.x86_64 16.12.2-1.fc25
kf5-krunner.x86_64 5.31.0-1.fc25
kf5-kunitconversion.x86_64 5.31.0-1.fc25
kf5-libgravatar.x86_64 16.12.2-1.fc25
kf5-libkdepim.x86_64 16.12.2-1.fc25
kf5-libkleo.x86_64 16.12.2-1.fc25
kf5-messagelib.x86_64 16.12.2-1.fc25
kf5-pimcommon.x86_64 16.12.2-1.fc25
kf5-syntax-highlighting.x86_64 5.31.0-1.fc25
kf5-threadweaver.x86_64 5.31.0-1.fc25
libical.x86_64 2.0.0-8.fc25
xapian-core-libs.x86_64 1.2.24-1.fc25
In order to clean this up messagelib will have to be split up (instead of the -DMIMETREEPARSER_ONLY_BUILD=TRUE build switch).
We can also check whether the upstream packages can already split this up in packaging.