This is a stalled item from the ad hoc OpenType Working Group (the closed group working on OT layout that emerged from the April 2014 meeting).

The idea is to spec a new version of the BASE table to contain script and language system vertical metrics, corresponding to the three OS/2 TypoAscender, TypoDescender, and TypoLinegap, and using the existing OTL script and language system tags framework. The goal will be to provide suitable linespacing for different scripts and languages within single font, e.g. deeper descender values for Arabic script, taller ascender values for Vietnamese language to accommodate stacked diacritics.