Apple can still keep the similar design they've had so far and increase the screen size, the only problems for them would be:
A) do they keep the same resolution
B) do they increase the resolution to keep the same or similar Pixels Per Inch ?
IMO, they will eventually need to increase the screen size, 4.3" screens are the norm now and 4.7" screens are the screens found on the top of the range phones.
and technically this will be the 6th generation iPhone, so calling it the iPhone 5 would be a cock up, they could get away with 4GS IF it looked like the 4/4S and had 4G connectivity - but it'll probably be the "New" iPhone as they seem bent on ridding themselves of version numbers.