I had an idea I figure I would toss out there.
It would be nice if I could click on a button to add the cover image as the first page in a book file - in particular PDF's (where I find the need more often then epub or mobi etc - those tend to have the covers more often already - whereas PDFs quite often don't).
This won't make a difference within calibre - but it will on devices where the first page is shown (and thus the cover makes it more easy to see and find).
So I was just thinking out loud here - if anyone has any thoughts.