> One feature that would be cool is an Excel addin to which you would designate a bunch of cells from a spreadsheet as inputs and one output, would follow the logic and emit code to replicate this logic as a function.

From having "you suck at excel" run in the background a few weeks back, I learned that feature is already in excel, there are "input", "calculation" and "output" cell formats/styles: https://youtu.be/0nbkaYsR94c?t=2756