Just got my Air yesterday after upgrading from an iPad 3. It's OK but I'm not as impressed as I hoped I'd be. I think I fell for some of the hyperbole in some of the reviews. It's all left me feeling that maybe Apple judged the Air quite well and have left plenty on the table to make the iPad 6 a very worthwhile upgrade. My guesses are:
Cameras - no idea, I'm not really interested in that stuff on my iPad.
SoC - A8 presumably. Personally I hope that Apple stops trying to double performance every generation and starts aiming for more modest 20% to 30% speed increases but with very significant power reductions. The only area where I think performance should still be a focus is the flash memory bandwidth. My Air really doesn't feel that much snappier when starting up applications compared to my iPad 3, and lots of people say the 3 was a dog (although I thought it was fine). I suspect that for starting up apps we've got to a point where loading the executable off the flash storage is a significant bottleneck and improvements there could be very noticeable to the user.
RAM - I think they'll go to 2GB in the iPad 6.
Display - Same size and resolution but maybe a switch to IGZO screen to reduce power consumption.
TouchID - Yes. I think they'll add this in 2014.
Battery life - This is more of a hope than a prediction but I hope that, even if Apple reduces power consumption a lot with a new A8 and IGZO screen, they only look at maintaining or giving a very slight 10% increase in battery life because...
Form factor - My main reaction to my iPad Air is that those reviews saying it is a real "wow" moment to pick up an Air and feel how light it is are exaggerating. The weight reduction is great but in my opinion it still needs to lose another 100g or so of weight until it really feels feather-light. I hope that Apple will continue the weight reduction in next year's iPad with another significant drop. After playing with my Air I disagree with the negative threads here that say the Air is now too thin and feels flimsy, it still feels completely solid to me and I hope that Apple can shave another 1mm or maybe more off the thickness next year while keeping the same rigidity. A reduction in the size of the top and bottom bezels might also be a possibility since those thinner side bezels do make the screen seem a bit bigger because they focus attention on it a bit more and any size reduction also helps with portability and weight.
All the above put together would definitely have me buying the iPad 6 next year, especially if they make the base model a 32GB unit.