I looked at a lot of template sites as well, and the thing that I found was an issue was cross-browser functionality.
I also didn't find a lot of fluid layouts, even in the ones that used CSS. I finally decided to just learn to do it myself with a lot of help from the CSS forum here, and a bit of hired expert help.
There was one really good fluid CSS template done by a guy in the CSS forum, it was called Iris-something. He offered it for free, but I would have paid him for it if I could get the color scheme I wanted and incorporated my logo and graphics. But I was too hurried to do the color testing.
So..to create a perfect template, I'd say:
Easy to customize color, (say with the divs marked, bordered, and numbered so the uninitiated can clearly see WHERE the applied color displays)
varied places for graphics
Optional menu locations
Optional "Pull quote" divs within a content div would be wunnerful.
To really sell, an added template for HTML mail that would not use CSS, but would complement or match the website would be a great feature in a template package. I'm thinking that alone could up your asking price considerably.
Those things would make a customer out of me if I was looking for a turnkey site layout. Now, if you can do all that, call me :)