Apple iPhone 5 rumors have been running around for months, and the delay of the release of the new phone has only made the rumor mill run harder. I don't think Apple has to do all that much to make its next iPhone a success.
Bigger camera. A 5MP camera just does not cut it anymore these days. The Sony CEO stated several months back that Sony will be building its excellent camera for the next iPhone. Expect at least a 8MP unit.
Larger screen. The iPhone 3.5-inch screen is really starting to look small, but I do not expect Apple to follow the Android manufacturers lead with monster 4.3 to 4.5-inch phones. A nice incremental increase to 3.7 to 4inches would a good step for Apple, giving a larger screen and maintaining the handy size. If Apple built a monster screened iPhone, it would have to release its top of the line product in two sizes, one with a smaller screen. Not everyone finds handling a phone with a larger 4.3 to 4.5-inch comfortable.
Dual core processor. Putting in the same 1 GHz dual-core ARM Cortex-A9 processor and PowerVR SGX543MP2 GPU as the iPad 2 will be enough to keep user happy for the next two years. Putting this in a phone the size as the current iPhone will be an amazing technological feat on its own. No one has put a dual core processor in a phone as small as the iPhone.
LTE/4G. Most of the world does not need LTE or 4G yet, but if you are picking up a new phone to keep for two years, I think this in one feature many people will be looking for. If not, it will be deja vu. In 2007 when the iPhone launched without 3G.