They seem to oscillate, tick-tock, like Intel.
1 to 3G similar internals (same CPU, new cellular), new externals
3G to 3GS big internal changes, similar external
3GS to 4 similar internals, big exterior design.
4 to 4S big internal upgrade, similar external
4S to 5 big all around. (outlier, should not have had such a major SoC upgrade)
So 5S/6, what's in a name, who cares? Lets say the exterior stays similar. What happens internally? SoC? It will be time for the A7.
2010 A4
2011 A5
2012 A6
2013 A7... quad-core doesn't play out as necessary for mobile devices. I predict more efficient dual core. Maybe they go CortexA15, but probably they'll go with another evolution of Swift. If they go quad-core, it's because the design team failed to execute on a significant evolution of the Swift and they had to brute force it with quad-core which won't be near as energy efficient. Then expect tradeoffs to maintain battery life.
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Lightning to thunderbolt/usb3 support!
Thunderbolt ports are too few and far between.
USB3 isn't necessary unless they upgrade their internal Flash memory to transfer at that speed. Most people use WIFI, WIFI backup when charging on nightstand, iTunes Match.
The cable is drifting away. It may happen, but it doesn't seem like a worthwhile place to dedicate engineering hours.