How about a kickstand to make it easy to view movies hands free? Maybe a way to redirect audio so instead of projecting sound down, it is projected towards me? And make sure you offer it in lots of colors.
How about all of the above? That's what Bandshell ($29.99) has done, with a nice case that also has an integrated kickstand, similar to my Microsoft Surface tablet and several Android phones. The Bandshell also has a novel (and patented) slide-down deflector that redirects audio to the user, as Apple should have done in the first place.
There have been other phone cases that redirect audio, but they tend to add bulk. The Bandshell's retractable design makes the case no bigger than most protective cases, and the deflector only adds to the length when in use. In fact, I've found phone calls were clearer to the recipient, as your own voice is redirected into the microphone, similar to the way it was with the old flip phones.
The Bandshell has been able to find a niche and fill it. If you want better audio out of your iPhone, and a case with decent protection, and a kickstand to boot, check out the Bandshell.