I doubt that it's going to be a big spec jump in terms of CPU and GPU.
It'll probably go from SGX543MP3 to SGX544MP3. The 6 series of PowerVR are not available yet as far as I know.
As for the CPU, I don't even know. Isn't the iPhone 5's CPU based on A15? Maybe they'll offer a quad core model instead of dual core.
I hope they'll bump up the RAM to 2GB.
Maybe the storage will go up to 128GB with 32GB being a minimum.
I expect the camera go to up to 13MP. Galaxy S4's camera beats iPhone 5 convincingly, so Apple has to catch up.
As for the killer feature, frankly there isn't anything they could do that hasn't already been done on Android. NFC, finger printer reader, temp sensor, etc... it has already been done.