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Cameras: Increased Megapixel front and rear iSight, with True Tone Flash
It goes without saying that Apple is likely to use a higher-megapixel CMOS in the rear and forward iSight cameras. It would not surprise me to see 8MP in the rear and 2MP in the front, particularly for newer augmented-reality apps as well as high-definition Facetime and possible facial gesture recognition and security enhancements to Siri and iOS 7.x.
Additionally, I expect some improvements in the lens elements to allow for improved image capture in lower-light scenarios, as well as improved image stabilization software and incorporation of the iPhone 5s's True Tone flash.
System on a Chip: A8
The A7's 64-bit architecture as introduced with the iPhone 5s and the iPad Air was a real shocker, and it leads us to beleive that Apple is heading down a path of platform convergence with their computing products.
However, with Apple facing a lot of competition now from Samsung's home-grown high-performance Exynos 5 processor used in the Nexus 10 and the Galaxy S 4 smartphone, we're likely to even more disruptive improvements in Apple's own silicon with the introduction of the A8.
While the use of a Quad-core SoC design in the iPad Air 2 is likely, we may very well see the A8 conceived as a "hybridized" chip using custom silicon, which might utilize a combination of two higher-clocked 64-bit cores and two or more smaller, lower-power and slower 64-bit cores, so that applications will run on a "cluster" of cores best suited to their workload, thus making the SoC more energy-efficient.
This hybrid core technology, known as big.LITTLE and licensed by ARM Holdings, could very well make it's way into the A8 in the iPad Air 2 and iPhone 6. Samsung has already announced its own big.LITTLE chip in the form of the Exynos Octa which is used in some international carrier (GSM) versions of the Galaxy S 4.
It also goes without saying that we are also likely to see a 2x or more boost in GPU processor power over the A7, increasingly moving the needle closer to the capabilities of their PC and console counterparts and enabling the most demanding games on tablet hardware. An 8-core GPU design using PowerVR Series 6 technology is not out of the question.
Networking: Dual-band 5Ghz 802.11ac / Low-power World Band LTE and Bluetooth 4
The iPad Air saw an improvement over the built-in wireless networking capabilities of the New iPad, the iPad 3 and iPad 2 with a dual spatial stream, dual antenna 802.11a/b/g/n transceiver. This effectively doubles the previous generation's wireless networking speeds from 65Mbps up to 150Mbps, provided the Wi-Fi infrastructure and broadband connection supports it.
We expect that Apple will eventually move to Qualcomm's latest WCN3680 Atheros 80211ac-compatible chipset which will allow the iPad and presumably the next version of the iPhone and Apple TV to communicate using anywhere between two and four spatial streams, enabling the device to transmit and receive data up to 300Mbps to 450Mbps, and with longer range and less power consumption than its predecessors.
802.11ac capability would of course require upgrading to an 802.11ac-compatible wireless router as well as updated Airplay-compatible devices in order to fully take advantage of the new chipset, so presumably a new 4th-generation Apple TV and a new version of the Airport Extreme is also in the offing.
The upside of this increased speed will be much smoother playback of 720p and 1080p streamed Airplay content from the device as well as more responsive, higher-fidelity screen mirroring.
An updated Qualcomm Atheros chipset in the iPad Air 2/iPad mini with Retina Display 2 and iPhone 6 would also mean updated, more power-efficient world phone + multicarrier LTE capability as well as an updated, more power-efficient Bluetooth 4.0 implementation as well.