At present all lines are drawn at the same thickness. However, some people such as partially-sighted users require particularly thick lines. GNOME bug 86040 suggests that the line format be extended to give the width as a fraction of the current icon height.
Since this is a change to the format, it must appear first on a branch. This bug actually predates v2, but wasn’t included in that version because it wasn’t marked as blocking the v2 tracker bug.
If this would still be useful to anyone, it could certainly still go into v3 of the theme format.
This bug is perhaps the first place where the use of SVG was suggested as a workaround for having to add new features. Elsewhere, someone also suggests a novel idea of allowing themes to ship TrueType fonts which contain unusual glyphs, in order to have access to another well-defined vector graphics language. Of course, this would be subsumed if we decided to use SVG.
Photo © Callistobreeze, cc-by-nc-sa.