As someone who is often laying out text in large booklet type documents, I often need to adjust text and leading on text to visually suit the design Im working on. An annoying function Ive run into (definitely not the end of the world) is that if I may need to adjust the size of the font and the leading of it to suit what I need... If I then adjust the size of the text, the 'custom' leading size doesn't constrain to the size of the font as you increase it.

This would be an awesome feature to be included, and would save a bit of time when working with fonts and text sizing.