Amusement du jour: we're finally seeing 64-bit ARM chips coming out, both in the iOS and Android universe. Those same chips, of course, could be mounted in a MacBook Air case and you'd have an entirely reasonable device that would still be a "real" Mac, albeit perhaps without needing a fan. That would be all kinds of attractive.
I doubt Apple will do this. The Air 2 shows their intent to push the pure tablet forward as a computer. For those who want a keyboard, they can easily add one via 3rd parties.
A MacBook Air running an ARM processor would force the Mac to support 2 architectures with some strong pain points. x86 code would need to be somehow runnable on the MacBook ARM Air, or it loses all the 3rd party apps out today.
I think if the Surface ARM units had taken off in a meaningful way, Apple may reconsider. The Windows market didn't pick up on Windows RT though, and it looks like Surface RT is dead in favor of Intel based Surface Pros. I don't see Apple doing anything major in this space to improve on what Microsoft attempted.