For the most part, the non-set items that drop in game come from a very small set of motifs: base racial, a fair amount of primal, barbaric, maybe a bit of ancient elf.
I'm not sure if Daedric drops anywhere or not, but I haven't been systematically looking at it.
The two exceptions that I have noted:
Vvardenfell (and HoF) drop non-set items in the Ashlander motif. I was reminded of this during the recent Tribunal event and got curious.
IC (where there aren't any set items dropping anyways) has a fair number of items that drop in Xivkyn.
Are there any others that people have noticed and remembered?