The iPad Air is a great device but even brilliance can be built upon, so there's room for improvement for Apple's sixth generation slate - aka the iPad Air 2.
Since the Air shares much of its design and many of its components with the latest iPad mini, some of this list overlaps with what we want to see for the iPad mini 3. But given the new iPad Air will be the flagship model, there are a few other things Apple could do.

A bigger screen

There's always a tradeoff between portability and screen size, but the success of some competing tablets shows that there is a market for devices larger than the iPad.
And before you dismiss the idea, remember this is the company that said it wouldn't build a smaller iPad and then went on to design the wildly successful iPad mini.
iPad Air 2
Recent rumours have raised the spectre of a 12.9-inch iPad, significantly larger than the current 9.7-inch screen and running at 2K or even 4K resolution.
While the current Retina display running at 2,048 x 1,536 pixels could arguably already be described as 2K or very close, having a physically larger screen would be good for movies and photo editing, and Apple likes to boast about statistics when it suits.
Of course, this could actually be the much-vaunted iPad Pro, which would mean the iPad Air would stay the same - and if that's the only way to maintain portability, we're all for it.
Current rumors are pointing to a more 'pressure sensitive' display, which would allow for better reception of prods from your digits and even that much-fabled iStylus that could do everything but make the tea for you.

Bluetooth mouse support

As the iPad gets more powerful it becomes a more realistic proposition as a work machine, with current models inching closer to being dubbed true laptop replacements. The only problem is that touch input just isn't as accurate as mouse input for some tasks.
Adding support for mice or graphics pen-style devices over Bluetooth could make the iPad a more usable device for more complex tasks, especially when it comes to graphics apps.
This would be quite a step for Apple since it touts touch as easy and intuitive, but it would nonetheless be welcomed by some users.

Facial recognition

iPads have had a front-facing camera for ages, and the Camera app is already able to detect faces when focusing.
So it would be nice to have some means to unlock or authenticate your device by simply holding it up as you normally do and having the camera recognise you. However, there's a big caveat here: Apple needs to do what it did with Touch ID and make the system work instantly, and be very hard to spoof.
There are other possibilities instread, such as eye tracking, scrolling pages depending on your eye movements, or pausing movies when you look away. Again, we've seen these already, so we'd be looking to Apple to make the system flawless.

Touch ID

Like the Mini, the iPad Air still lacks the Touch ID sensor found in the new iPhone 5S. This seems a little odd for an expensive flagship device, and in future you can expect to see fingerprint sensing used for more than just unlocking a device, so putting it in the iPad would enable easier downloads, logins and even user switching.
iPad Air 2
Its current omission may be a cost issue, in the sense that Apple wants to protect its profit margins, but over time all technology tends to become cheaper so it would be a welcome inclusion.

A card slot

Look, we know that a lot of people reading this will laugh in our faces, but hear us out. We know there's no way Apple will put a card slot in its sleek, machine-tooled surfaces of its iPad but there's no doubt that a way to bring content more easily on and off the tablet is needed.
With games and bigger music and video apps easily eating up 1GB or more, the base 16GB iPad can quickly fill up, especially if you're going to be away from your computer and unable to re-sync new content easily.
So if it's not a card slot (which it won't be, unless the moon suddenly turns into a banana and gravity inverts) then some other ability to sync content - be it a digital locker, a Lightning port connector with card slot that can do more than just photos, or some other idea that multiple intelligent bods can dream up.
Come on Apple, we're not doing all the work for you.

A haptic screen

Touch is pretty amazing though we all take it for granted now. An interesting way for Apple to go could be to use a pressure-sensitive haptic screen in its iPad lineup.
iPad Air 2
As well as providing better interaction for painting and drawing apps, it could surely leverage the extra input information for other purposes like pressing hard on a link to open it in a new tab.

A file browser

This is more of an iOS wish than specifically an iPad one, but iOS currently provides no real access to a file storage or manipulation area.
Sure, third party apps do this, each in their own way but if Apple implemented it at a system level it could standardize the process of downloading files (not currently supported by Safari), saving documents and storing stuff so that all developers could provide a better overall experience - and would facilitate expandable storage in some form, as we mentioned earlier.
Apple wants you to use iCloud to sync all this kind of stuff but that presumes that you always have a network connection and that users will happily pay to upgrade the measly 5GB of standard iCloud storage associated with each Apple ID.

A quad core CPU

It's true that Apple's dual core A7 performs better than most quad core mobile CPUs in everyday use and this is down to the fact that by making the processor and the software and tightly controlling both, it can optimize everything very well.
But just imagine that level of expertise being applied to a quad core CPU. Track counts and frame rates would go up, render and load times would go down - it's going to happen at some point, and the boon in creativity, already one of Apple's taglines to buying it tablets, would be immense.
So a quad core iPad seems likely, though the timing is less clear. The physically larger battery available would mean the Air likely went quad core before the iPad Mini ever could - but would Apple really want to switch out its strategy?.

A better camera

The iPad's two cameras are good, but there's no real limit on how good people would like them to be.
It's not just about screen sizes, since the Air already shoots in HD, but improving the quality of the image, the zoom and stills performance would all be welcome and seem likely contenders for inclusion in future versions of the iPad, since camera technology is constantly improving.
iPad Air 2
And by the way, we don't mean the rear camera. That can go down to 2MP if it would make the design better, as it might discourage the scourge of tablet photography. We mean the front camera, improving it for use as a video conferencing tool and taking self portraits for social networks.

Live multitasking

By live,we mean the ability to have two apps on screen at the same time. Imagine streaming iPlayer while checking your email, and not having to switch out of one to see the other.
There would have to be limits, like not allowing two apps to play sound at the same time, but it seems plausible enough on the larger screen of the Air, and with its significant processing power.
Again some other manufacturers have tried this with limited success, so it would be up to Apple to show them how to do it right - ideally it would be contextual or even Siri-driven, allowing you to be working on a spreadsheet or watching a movie and say 'Siri, is my train delayed?' and have what you were doing staying front and center with the information subtly popping up.