1) Larger display
This is almost certain to be a new feature in the iPhone 6. We have already seen leaks of prototype models and screens, so this is essentially confirmed.
There have been complaints that the iPhone's display is too small for years now. Even after Apple increased the screen from 3.5-inches to 4-inches in the iPhone 5, there were still complaints from people wanting a screen comparable to Samsung Galaxy or HTC smartphones.
This year Apple seems to be taking note as rumour after rumour suggest that this year's iPhone will ship in two all new sizes. The smaller model will feature a 4.7-inch display; whereas the larger model will feature a massive 5.5-inch display which is about the largest a manufacturer can go without crossing the phone/tablet line.
It has also been heavily reported that Apple has been investing huge sums of money in a company called GT Advanced in order to make the iPhone 6's display from sapphire crystal. Currently the display is made from Corning Gorilla Glass but a move to sapphire crystal would make the display pretty much impossible to scratch. It's already used in the camera lens and home button of the iPhone 5s, so it's very possible it could take next step up.
In the past every new number has brought a redesigned iPhone and there is no reason why this time round it should be any different.
Following the above link will take you to a few shots of the supposed iPhone 6. Apple is reportedly bringing back rounded edges for a more comfortable feel (yet maybe slightly slippery) when in hand. Also obviously to accommodate a much larger display you need a larger rear, so Apple is also redesigning that using all aluminium.
Buttons seem to be changing too; the sleep button is moving to the right hand side to make it easier to access with the larger phone. Also the volume button look like they are being replacing with longer and thinner buttons like those in the iPad.
3) Better camera
Improvements to the camera are now standard in every iPhone upgrade whether it be 4s-5 or 5-5s, every generation iPhone has in some way a better camera than its predecessor.
There hasn't been much rumour in the way of increased megapixels but as we all know it's not all about the pixels when it comes to creating a brilliant smartphone camera.
So it could still retain the 8MP shooter of the iPhone 5s, but it may have a new feature called Optical Image Stabilisation (OIS). This would react to the movements and motion of the iPhone using its gyroscope and create much steadier and crisper images, removing any shakiness and blur.
4) Improved processor and memory options
The iPhone 6 will more than likely feature a 20-nanometer A8 processor, which could offer 30% more performance over the A7 chip found in the 5s and iPad Air.
Apple's A8 chip should include a quad-core processor and quad-core graphics, but will probably still only have 1GB of on board RAM.
What hasn't been rumoured but what we've been desperately hoping for since the iPhone 5 is for Apple to drop to 16GB model and make the 32GB iPhone the cheapest option to purchase.
The problem with iPhones is that they do not have expandable memory so what you buy is what you have for as long as you're keeping it. 16GB is way too little memory for all the things you can store on your iPhone nowadays.
With all the improvements that each iteration of iPhone brings there will ultimately be increases in the size of things like pictures and applications, and the 16GB version makes it too difficult to manage all of that. So doing away with the 16GB and dropping all the other memory options down to the smaller ones price would be a welcome improvement.
5) iOS 8
Earlier this month Apple announced their next big mobile operating system upgrade with iOS 8.
The iPhone 6 will launch after the release of iOS 8 so will definitely ship with the new OS. Tools have been introduced which allow for a more seamless transition between all of your Apple devices. There will also be things like messaging improvements, interactive notifications, keyboard improvements, iCloud Drive and a new app called health.
For a more detailed look at iOS 8 click on the link above.