IMO this discussion is much less relevant to KVM-Whonix per: https://www.whonix.org/wiki/FAQ#Why_is_KDE_.28big.29_the_default_desktop_environment.3F_Why_not_use_a_minimal_DE.3F
Choosing KDE is a personal preference by Whonix developer Patrick Schleizer. KDE has one advantage, the only developer likes it and remains interested to maintain and develop Whonix further.
(Replace Patrick with HulaHoop and Whonix with KVM-Whonix. Whatever keeps KVM-Whonix going is the right answer here I think.)
This is also not a big deal for Qubes-Whonix since dom0 will handle all the Window Management and we're really just talking about including a set of common core apps (terminal, file manager, text editor, ?)
So the main impact DE choice will have is on Virtualbox-Whonix. If it's true that advanced users have all migrated to KVM & Qubes, then the Virtualbox-Whonix platform should especially cater to users new to Whonix and also, new to Linux. The urgency is to find a replacement for KDE5 before Whonix adopts Stretch. (Are we sure that KDE5 can't run well under Vbox?)
The FAQ (linked above) is outdated. MATE is now included in Debian repos: https://wiki.debian.org/DesktopEnvironment. And seems non-KDE/GNOME choices have gained somewhat in popularity.
Made this list yesterday (from lightest to heaviest):
- LXDE: what @HulaHoop said re: LXQt. Actively developed and has a captive userbase in low-low-resource systems (netbooks, SOCs). Can't comment on usability. If it's light enough, could it power Vbox-Whonix and also be included in Qubes images as well? Then, just add some Gnomish apps on top of it? KVM-Whonix should be able to throw KDE on top of it with ease. I'll try it out next.
- MATE: fork of GNOME2 updated to GTK3 libraries. Familiar desktop, popular? My concern is that GNOME2 is no longer maintained and my guess would be that many apps forked over to MATE are simply cosmetic re-skins - meaning MATE won't benefit from GNOME team's future work (esp security-related). Do they have the manpower to maintain such a big DE?
- Gnome-Flashback: GNOME2 look-alike based on GNOME3/GTK3. This design seems preferable to MATE but does anybody use it? I can't find any numbers of DE-users for this or any distribution anywhere... Still, worth looking into. Have to check if installing KDE apps on Gnome requires more dependencies or if the converse makes more sense.
- Cinnamon seems to be hugely popular (see Linux Mint). Based on GNOME3. I tried this out and it is indeed highly pleasing aesthetically and usability-wise. Unfortunately, it felt just as heavy as GNOME3, even running under a distinct software rendering session.