Apple iPhone 6
The presumptive name for Apple's next-generation iPhone is iPhone 6. Yes, it will be the 8th generation phone in Apple's product lineup, internally referred to as iPhone7,1, but don't let those numbers distract you. Apple can call any phone it releases by any marketing name it wants and the next number after 5/5s is 6. So, iPhone 6.
iPhone 6 release date speculation
If Apple holds to the same pattern as the last two years, the iPhone 6 will be announced on or around September 9, 2014, with a release date of September 19, 2014. That said, Apple doesn't always hold to patterns. There's never been less than a year between iPhone releases but there were some 16 months between the iPhone 4 and the iPhone 4s. There were, however, only 6 months between the iPad 3 and iPad 4, so no release date is confirmed until Apple announces an event and tells us what it is.
Update: September 9 is gaining momentum as the iPhone 6 announce date.
iPhone 6 screen size rumors
For a long time now rumors have swirled about Apple going to a larger display size for the iPhone 6. The original iPhone had a 3.5-inch 480x320 standard density display. The iPhone 4 took things to 960x640 double-density "Retina" and the iPhone 5 to 4-inches and 1136x640 double-density. That's a pattern of steady improvements every two years on the two years, and if Apple sticks to it, we're due another. If the rumors are true, both 4.7- and 5.5-inch models are coming our way this year.
Update: The 4.7-inch model is sounding more and more likely while the 5.5-inch model is sounding like it might follow along later.
Questions remain as to resolution, with @3x — 1704x960 — rumored at the low end and 2K — 2560x1440 — rumored at the high end. Also called QHD, 1440p phone from other manufacturers are already starting to hit the market, so while insane, it's doable. New, scalable interface tools have been added to Xcode 6 storyboards, so anything really is possible right now.
If Apple stays conservative they could scale up the interface this year, add more pixels come iPhone 7 in 2016. If they still have their foot down on the gas, however, maybe we'll see a leap forward just as spectacular as 2010.
iPhone 6 sensors and specs
Keeping to the pattern theme, ever iPhone announced since the iPhone 4 has had a new, custom chipset announced along with it. That suggests the iPhone 6 should come along with a 64-bit Apple A8 system-on-a-chip (SoC). That would also mean improvements to the iSight camera's image signal processor (ISP) and even better photos.
The M7 motion coprocessor could likewise be updated to M8. Other manufacturers are including natural language and context coprocessors, so there's no telling what else could be done there. Code for barometers and air pressure sensors has reportedly been found in the iOS 8 beta, so new sensors could be along for the ride as well.
Touch ID is already great from a hardware perspective, but with Apple's PrimSense acquisition, perhaps Kinect-like motion sensing could be integrated into the iSight camera as well.
Speaking of which, Apple has also been working on cameras since the iPhone 4 and if they keep doing that, we should get more with the iPhone 6 as well. Some of that will no doubt be software, things like dynamic focus, but perhaps Apple could squeeze some better hardware in as well, especially optical image stabilization (OIS).
iPhone 6 radios
Apple already has one of the best, most compatible LTE radios in the world in the iPhone 5s. If anything, it can only get better in the iPhone 6. It's doubtful Apple would stoop to support dead technologies, like adding a second, battery-wasting radio for voice + data on Verizon's aging CDMA network.
802.11ac Wi-Fi, however, is likely. Apple already supports it on the Mac and with their AirPort line of routers.
Near-field communication (NFC) is again being rumored for the iPhone. Apple has tested it in previous years but decided to stick with Bluetooth Low Energy (BT LE) for proximity.
iPhone 6 health and iWatch
In conjunction with iOS 8 and the rumored iWatch, the iPhone 6 could also be part of Apple's next big push — health.
Things like counting steps are already commonplace on the iPhone. With the Health app shipping standard come this fall, it'll be interesting to see just how far Apple can take lifestyle and wellbeing on the phone.