SUMMARY
A thing I actually miss when I had to use Windows back in the day was Gmail Notifier Plus[0], a mini app that sat in the taskbar and notified you for each new mail, giving you the chance to cycle though all new emails, preview them and opening/deleting them. It was immensely useful since one didn't had the need to have a tab opened for gmail every time (or, know, kmail running) - you could pretty much manage your email from that mini app.

For a brief moment we had a kind of similar thing in the KDE4 days, whose name I cannot remember. Sadly, since KDE "5" and the migration from Python to QML plasmoids, that ability to interact with kmail was lost and, after getting to know a bit about QML since a couple of years ago, primarly due to not having any datasource/dataengine where to pull email data from.

With that in mind I think kmail (and Plasma, in general) are lacking a proper and more complete email notification system. The current one works fine for a single email (well, not very much at this moment[1]) but the thing with the QML datasource/dataengine handicaps any possibility that even a iliterate programmer like me can provide an option for that.

So my wish is to have QML datasources/dataengines for more of the stuff Akonadi handles - not only emails but akonadi feeds, contacts, reminders and the like.

ADDITIONAL INFORMATION
[0] https://github.com/shellscape/Gmail-Notifier-Plus
[1] https://bugs.kde.org/show_bug.cgi?id=455861