The iPhone 4S is truly a remarkable device, and there's almost nothing it can't do. The only thing I would like would be a slightly larger Retina display but not at the cost of pixels per inch -- there is no other display like the Retina display. Some come close, but close doesn't count. I find it hard looking at my 24" iMac or 24" Dell Ultrasharp monitors afterwards, which are also IPS. The Retina display gives us a small taste of what 4K video is probably going to look like when we start seeing it towards the end of the decade. (If you ask people what they would prefer, 3D or 4K, most will say 4K.)
I'm not sure why so many people want a thinner phone -- so they have less room on the sides to grab onto and drop them more frequently? The curved back of the previous generations was ergonomically better and easier to hold on to. I think the ideal iPhone would be a marriage of the two with a larger Retina display. I know people are wanting an edge-to-edge screen but I doubt it will happen. Most cases cover the edge of the glass and would therefore cover some of the display.
I'm pretty sure Apple already has the iPhone 5 designed but they are probably holding back until they can perfect the power management aspect of LTE. People who use 4G hate the short battery life they get as a result and Apple likely won't compromise on something as important as that.
Have you heard of the Galaxy Nexus? It eats the 4S's lunch.