Even before the iPhone 5S is out, analysts are talking about the next iPhone 6 for 2014 and this is what they have to say.
Last year's iPhone 5 was an evolution over the previous iPhones and this might be the case with the upcoming iPhone 5S as well. The new 5S is reported to come with multiple color options (taking cues from Nokia it seems), an upgraded quad-core Apple A7 SoC, a higher resolution 13 MP camera as well as a fingerprint scanner (if the technology is ready and working on time). However, these features don't really excite because the basic package will still remain the same, i.e. the design and the display size and even the screen resolution will most probably remain untouched.
Enter iPhone 6. According to analysts Apple will introduce this new generation in June 2014. While we're 13 months away from said time, analyst reports give us much hope as they fully expect Apple to embrace a bigger screen size of perhaps 4.5 inches (reacting to market trends after all), and along with a higher resolution display to keep up with the Retina status (that they "invented" in the first place).
Obviously by just putting a larger display you can't have a better phone. Software plays a huge role in it and iOS 8 will play such a role, similar to how iOS 5 was built to support the bigger display, widescreen aspect ratio and higher resolution found in the iPhone 5.
There have also been reports of the new iOS 7 featuring a redesigned UI. A source site describes the design of the new UI as
"The interface loses all signs of gloss, shine, and skeumorphism seen across current and past versions of iOS."
The redesigned UI will be "flat" to the point that it' remniscent of Microsoft's Windows Phone. Now we don't know that if that's a good thing or bad (for iFans) but a new design would really be a refreshing change as it's only been about 6 years since we were introduced to the original (and almost identical) UI of now.