I am satisfied enough with my KAO, but it is not perfect, so my perfect Kobo is:
8" or 8.5" screen, perfect light with comfortlight and 190 g.
Kepub with ligatures so I could abandon Epub.
I don't care about buttons but I have never had them so may be I could like them very much. And I do not care about sound, it can be better done with a phone or a tablet.
All the patch we install should be options in the main firmware (I know that settings would be enormous but at least
the ones I use should be there 
) The same for hidden features.
The quality of touch should be much better than KAO because it is terrible. Also for wifi (on my KAO it is not good). Faster software without lag when You touch it in choosing books or settings (turning page is not so bad). And a decent pdf handling, just like KOReader.