I have a soft spot for Window Maker and I have been using it until recently , when I moved to Wayland-based environments.
I also really like its dockapps (I even wasted some time writing one to display and control screen brightness), the only weak point for me is the systray dockapps, probably because it is a concept “transplanted” from other desktop paradigmsthat does not fit great.
Personally i'm not that invested in what NeXT came up for their DE, i don't even think they were that great from a UX perspective (i used Window Maker long before i even learned about NeXTSTEP and once i used one i found its window management way inferior to Window Maker), so i don't mind the systray stuff. I use a dockapp (wmsystray) that "splits" the icons into pages with (IIRC) four icons per page and little arrows below.
The only thing i'd like (and i might do and try to submit a patch at some point since AFAIK it is still being maintained) is for it to put a small frame around the icons like some other dockapps do for their contents as right now it just displays the icons on top of the dock tile background. But that is very minor.
FWIW NEXTSPACE isn't just a Window Maker fork but an attempt for a complete NeXTSTEP-like desktop environment based on GNUstep and Window Maker is only for the window management bits.
And since GP mentioned NeXT, it should be the perfect match. Window Maker is based on the NeXT UI.