We won't know until they're officially announced, but it's worth looking at some of the WP 8.1 features in order to get an idea. For starters, it wouldn't surprise me if we see hands-free prompting (akin to Moto X) for Cortana, but that would require specific hardware to achieve. At the minimum, we should expect the Lumia 930 and other higher-end devices to be tuned for their CPU and RAM from the start. Why is that important? Well, with the new generation we can expect quick shutter times and loading between shots using the PureView camera; we can be assured that Miracast and WiFi Direct will work without hiccups, etc. That said, the Lumia Icon and 1520 are up there in terms of specs, you should expect appropriate firmware updates down the line. Either way, you're going to wait (i.e. for next gen WP8.1 handsets or L1520/Icon firmware updates).