I’m relatively new to Glyphs, so this may already exist, but here goes.
When designing a Black weight, a lot of glyphs (such as fractions, percent, and other fiddly symbols) become very dark, so I make them as dark as possible while maintaining some degree of legibility. As a result, I often want to use the same outlines in the extrabold/bold as in the black. Currently, I can fix that with the brace approach, but I have to manually go through all denominators and numerators, and make sure the spacing and kerning in my ‘intermediate’ layer is correct.
The simple solution:
What if I could just draw an interpolation line that moves diagonally upward from the regular to the bold, and then horizontally from the bold to the black (to clarify: a ‘normal’ interpolation line would go diagonally all the way). Then, I could simply choose to apply this line to any number of glyphs. So basically, it just allows to interpolate some glyphs along a different ‘line’ then others.
Hope this make sense. Cheers!