Coherent theming, although you've hardly had that since Windows 98.
I've applied themes to make Xaw, Qt, and GTK software more Motif-like, but the GTK ones seem spotty and the Qt theme doesn't work for Qt6, and fonts are inconsistent.
If you want a coherent motif-ish theme, NsCDE is amazing. It themes like everything in the world and is honestly like the most consistent looking desktop I've ever used
I tried pulling in the theming from there, and while it works miracles, I still want to do the three-headed dragon meme:
Real Motif apps
Qt5 apps (where there's a Motif-like theme baked in)
GTK apps, which don't honour the same fonts and the theme is far more divergent from the "real deal"
There are a few other "Solaris 9" and "Perl Tk" lookalike themes that also come close, but they're all sabotaged by GTK's lack of bitmap font support (The old bitmap Helvetica is my go-to UI font)