Apple's (NASDAQ:AAPL) iPhone 6 is barely two months old, yet rumors about the device's successor are already swirling. If previous handsets are any indication, Apple's next smartphone should feature a similar body, but faster processor and a radical new feature designed to entice buyers.

The iPhone 5S was defined by its fingerprint scanner, Touch ID. The iPhone 4S introduced Siri, and the iPhone 3GS offered video recording. The iPhone 6S (assuming Apple sticks by its long-standing nomenclature) could continue the tradition.

A much improved camera
According to longtime Apple blogger John Gruber, the iPhone 6S could be defined by a radically improved camera, one featuring two lenses that would take photos on par with a DSLR.

It certainly wouldn't be surprising: Apple has placed great emphasis on the cameras in its mobile devices. The recent introduction of the iPad Air 2, for example, was dominated by an extensive camera demonstration, and the regular improvements made to the iPhone's camera have been a central talking point of nearly every new iPhone introduction.

The iPhone 6 and iPhone 6 Plus already offer some of the best cameras ever built into a smartphone, but they still pale in comparison to the DSLRs favored by more dedicated photographers. Given the steady improvements Apple has made to the iPhone's camera, however, DSLR-like quality seems only a matter of time.

"You don't see it in a lot of places"
The other rumored possibility is a 3D display: According to China's Economic Daily News (via BGR) Apple is developing a 3D display for the iPhone that would not require the use of glasses.

If so, it could be similar to's (NASDAQ:AMZN) much-maligned Fire Phone. That device, launched this summer, relies on a series of cameras to track the user's face and present a sort of holographic, 3D image. Unfortunately, it hasn't been enough to spur sales -- the Fire Phone has largely been a failure. An aggressive price cut happened almost instantaneously, and Amazon executives have been willing to admit that mistakes were made.

Apple's CEO Tim Cook was blunt in his assessment of Amazon's handset, telling Charlie Rose that "you don't see [the Fire Phone] in a lot of places." Of course, it's hard to imagine Tim Cook heaping praise on a competitor, but the failure of the Fire Phone makes this rumor a bit more difficult to believe.

To be fair, the Fire Phone has numerous other problems, and while consumers may find the 3D display attractive, its failings -- notably a lack of apps and its use of Amazon's esoteric Fire OS -- may be too significant to overcome. Apple doesn't have that problem, and its 3D phone (should it choose to release one) could find success where Amazon has failed.

Apple will need to entice buyers
Apple has yet to reveal the final numbers, but it appears the iPhone 6 has been a stunning success. Last month, when it reported earnings, the company revealed that it was selling everything it could make, and some models of the more expensive iPhone 6 Plus still remain in short supply.

The issue could be maintaining that momentum: With Apple finally introducing the long-desired, larger screen iPhone, buyers may have little reason to snap up future models. This is even more true given the growing popularity of subsidy-free, no contract plans: Although these plans allow subscribers to upgrade far more often, they also offer discounts to those who choose keep their phones longer.

In the past, Apple may have relied on the regular, two-year upgrade treadmill to keep its customers coming back. As subscribers move to subsidy-free plans, Apple will have convince them the latest model is worth their money.