After playing with CPMaker over the weekend, here are some of the features that I think would make it great.
1) Allow use even if -nobezel is on
2) Display with the proper aspect ratio
3) Support all controls including joysticks, player start, trackball, ...
4) "No controls available" notification (load a different picture?)
5) better control over label font (discussed towards the bottom of this page:
http://www.arcadecontrols.org/yabbse/index.php?board=3;action=display;threadid=26534;start=0)
Maybe not all of them are possible, and others can probably be implemented as "workarounds".
For example if Howard's Johnny5 turns out to do a better job at generating the picture, then maybe using johnny5/irfanview/CPMaker-display-only may produce better results than CPMaker only, at the expense of extra overhead when launching the game and no dimming support (unless Howard plans on implementing that in Johnny5).
Because of that I see 1 & 2 as the most important (and chances are, they're also the hardest), although I really like the "all-in-one" aspect of this app.
Edit: also having the ability to display some of the other controls.dat fields, such as "misc information" wuld be nice
