Stressed vs. unstressed vowel systems — German, English, Slavic.
Other dimensions than POA: phonation. Could tie this into diachronic effects of phonation, as in Khmer and some of those languages with no low breathy vowels. (What was the one in SEA?)
Vowels that don't quite fit into the triangle — diphthongs, apical vowels.
In-depth accounts of natlang vowel systems. I'll probably edit this