When making designspaces for variable fonts and profit you need to keep these basic rules in mind.
Some ideas for the construction of letterforms with overlaps. You can draw these letters in every possible way, the point is to think about what could be useful. Sloppily drawn so it easy imagine doing it better!
When interpolating you need to manage multiple versions of the same glyph. This page shows some of the common problems
From 2004 until 2014 Superpolator was the industry standard way for creating interpolation systems and large families. The MutatorMath library was open sourced, and all the functionality of Superpolator is now available in DesignspaceEdit2 for RoboFont. The Superpolator document evolved into the Designspace format, supported by Fonttools.