I have actually been thinking about this myself, figuring that a backplate has the potential to cool significantly more than it does, and perhaps the poor heat transfer of current materials used is partly to blame. I mean, take a full-coverage WaterBlock: you are cooling the front of the card, where all the big stuff is, but some heat will try and escape, and it will do this through the top of the card. Not to mention that the top has, in many cases, VRM's and/or VRAM chips that are essentially just sitting there roasting.
Another important aspect of a backplate to many people is its ability to hold the card in place, as no one wants a droopy GPU.
Lastly, of course, is aesthetics, and that is much more personal. I like the Heatkiller, EVGA, and Koolance Backplates, in that order. However, something I was thinking was that if the design did incorporate copper, it could use less of it if it were to have fins, similar to the Swiftech Heatsinks for use with the universal GPU block they sell. If the copper were only a few mils thick but had a greatly increased surface area due to a row of slim fins, it would be much more capable of cooling and would look very unique. Then, you could simply set a piece of stainless steel on top, with holes cut out for the raised copper, to protect the copper base, improve aesthetics, etc. You could even nickel-plate the copper fins so they don't clash with the SS so much.
I am just speculating here in terms of efficiency, and it's possible that it would have no benefit. However, I am almost certain that if you can design and manufacture a backplate that reduces temps more so than any other available model, and it looks even halfway decent, you will not be able to keep up with the orders!
Something else that seems like it would be useful, and unique, would be to incorporate a new PCI panel into the backplate, so that if you, for example, put one on a 680/670/7970/7950/7870 you can OPTIONALLY buy one with an integrated dual-slot or single-slot PCI bracket that is actually a piece of the stainless steel backplate that was cut and bent. I don't think that you could really get a much more stable mount than with a system like that. You could even offer them for air-cooled cards, which is a situation where the finned backplate might make even more of a difference, and make the PCI bracket a true high flow model, outperforming the EVGA model.
That would make your product unique in multiple ways, and considering how many people were desperate to get their hands on one of those hi-flow brackets a few months ago, if you can provide even 25-30 percent better airflow you will peak a LOT of interest. When they see that not only is the airflow improved, but the card is also actively cooled from the top/back AND this is all one solid piece you will be lighting your cuban cigars with hundred dollar bills.
Okay, maybe not, but you could probably light a Camel with a one-dollar bill a few times
I would draw out a rough example of what exactly I'm trying to describe, but my Dell Inspiron that I have been using until I finish my build has just gone crazy, and I have no f***ing clue what is wrong with it. I hate laptops, especially since you can't build your own like you can a desktop (although I really want to try; I would get a bunch of cheap laptops off craigslist and experiment, as I want a self-contained liquid-cooled laptop... I can dream I suppose). Thus, I have been posting from my phone for a while now.