I wouldn't say *all* they need to do, considering all the issues I've seen trying to use it over the past decade and a half.
For me, their multi-monitor support is still dysfunctional since the 4.0 days when I began using 3-6 displays. I use kde when I can on my laptop, but even when using an external laptop there, or my 3 displays on my desktop, it's still always really weird in a number of ways. Their kwin compositor really sucks at ultra high resolutions too, totally destabilizes the os over time. High res as in 3x 4k displays at 11520x2160, which is quickly becoming the new norm in displays, it can't seem to keep up and not leak itself to death.
I've been posting and arguing with the kde folk on
multi-monitor issues for years about some of the more annoying behaviors, and I'm not alone as the thread is quite long, and there are many others. I end up using cinnamon as a result now mostly. KDE4 they marked the worst issues multi-mon issues (refusing to save display arrangement settings!) as "won't fix", and still haven't made things much better in 5. Here's hope for future versions, maybe some new blood will give a damn to fix persistent core display issues and quit worrying about pedantic visuals.
My laptop is really odd - I can't even start kde on it since upgrading 16.04 to 18.04 ubuntu. Not taking the time to debug it, but using default ubuntu gnome, which I hate almost as much as Unity until I can fix it.
I'd love to hear from someone actually using kmail too. I've tried at least a half-dozen times over the years to use it in place of thunderbird, but it never fails to crap all over itself in broken ways, fetching, indexing, and in general just crashing with different imap services. I finally just got used to gmail in chrome with some extensions.
KRDC, a KDE app for windoze rdp and vnc client, still after at least 6-8 years will randomly change the entire key mapping during use. You don't really realize what is going on until windows change, things start closing, and random selections occur as you type. Then I realize I need to shut it down and restart the rdp session because it went stupid again. I assume I'm the only person still using it.
One thing I absolutely love that I still run under Cinnamon is KDE Connect for phone integration with an android client. Mostly because when I get a call, it mutes my music automatically, and can control my pc remotely. About the only reminder of KDE left I use that doesn't annoy me.
KDE and derivative apps are great in concept, but otherwise leaves much to be desired for long-term use with these nagging persistent issues that never get fixed, which using it off and on again over 15 years or so I think I can say with experience. So much potential squandered imho.
-mb