Other things they could add:
802.11AC: pending low-power, portable friendly chipsets
Up to 128 gigabytes of storage: definitely
Worldwide LTE support: supporting every band has some hardware implications, not sure if possible yet
NFC: waiting for something to emerge, I imagine
Haptic feedback: If something was ready for primetime...
Gorilla Glass 2: seem to be using their own brand of it
USB 3 and/or thunderbolt for faster transfer and charging through the port: USB not happening, especially USB3. Thunderbolt seems unlikely due to power draw and PCI interface requirements
Liquid metal body: what does this gain?
Faster touch detection like here: Absolutely needed but no shippable tech! As the guy in the video says, 1ms is his target for the next decade, which is a little unfortunate because the video does a really, really great job of showing why we need latency fixed more than a high resolution screen. (The TF Prime claims 50ms, though)
I think Apple's top priority is still power draw, which will come before almost any tech option they consider adding. Improved power draw is the gateway towards returning to iPad 2 batteries and frames (lighter/smaller), inevitable CPU/GPU improvements, and even the basic tech we expect to see like 802.11AC. Everything else, whether it be haptic or Thunderbolt or whatever, really depends on what sort of component cost and power profile they are looking at. They can't really afford to run hotter or heavier.