6. TextAreaX/Y
It's being able to find out the current on screen X & Y coords of a given bit of text (or the text cursor) in the text area. Useful for positioning a drop-down list of auo-complete options, also you need some Y info like this if you want to implement a gutter of some form to highlight things, possibly even if you wanted to do code-folding. Although by the time you get to the latter, you may as well have rolled your own gadget...or used scintilla...
7. Font Width
I just always felt it was a very obvious gap (FontHeight exists after all), and would be useful. I did play briefly with the fontconfig modules, but got in a bit of a mess from what I recall getting things installed and working.