I think it would be really usefull if we could use a .designspace to generate variable fonts. Also a more standard way of building them.
For one thing I feel that working with brace/bracket tricks is too much of an improvised and complicated way of substituting glyphs along the axis. Personally I think I would rather build my fonts outside Glyphs, then having having to work with those.
What do think about that?
I’m not sure what a designspace file would help. It still would need to be displayed in the UI and converted to the internal data model, otherwise Glyphs can’t do anything with it.
What do you don’t like about brace layers? They are just intermediated masters match those in designspace file.
There are some issues with bracket layers, I know. I’m working on that.
The thing that I don’t like is having the alternate shape “hidden” in a secondary layer inside a glyph. I would prefer to have them as actual glyphs, and then just assign them to specific areas in the design space. For fonts that have lots of substitutions this would be much more practical, and you would always see all the glyphs and their their different shapes on the glyph panel.
I would also feel more comfortable in having my font build in standard way. To make substitutions with these tricks is time consuming and if one day I need to build my font outside glyphs I would have to remake everything again.
I understand. But that has nothing to do with designspace files 
I was thinking in the same direction.
Yes, I realise that 
designspace files was the first thing that came to mind.
I just would love to have something similar what “Rename Glyphs” does as a costume parameter, but applied to specific areas of the design space, and working in variable fonts.