I don't believe it will be "The Biggest Jump ever." I think the Dual camera will see a minor revamp, perhaps with the zoom, portrait mode will likely be improved. But I certainly believe it will retain much of what the 7 Plus has now.
Apple is not stupid and they are not going to increase the pixels to the point where there is nothing but mush in the lower light images. A modest bump and lens improvement will be good with me. I love my 7+ camera so even if it was the same I would be happy.