That's never gonna happen.
If you look at Apple's history of screen resolutions (in phones, tablets, laptops), they always multiply the previous resolution by two. That's one of the reasons for the "overkill" in the iPhone's retina display. Anything above 250dpi (non pentile of course) is perfect to the eyes. The reason they got to 960*640 is that it was exactly twice the 480*320 of the previous gen iPhone. As for the iPad (2048*1536 instead of 1024*768). It's an easy way to keep compatibility with older programs, and avoid layout problems. The same happens when you run an iPhone app on an iPad (you can zoom x2 the application, and it doesn't even fill your iPad's screen).
So, I'm afraid that a new aspect ration for the screen would be a bit too revolutionary.