Compared to all of the other iPhones, the 6 line was a little overly-simplistic. I would expect to see at least a 4->5 evolution. The pattern hasn't existed long enough to really call it a pattern, but iPhone generations seem to go in groups:
2G, 3G, 3GS: rounded, chrome edges, plastic back, glass front
2->3 Changes: make the whole back plastic rather than the top half aluminum (material change), remove indentation on the 3.5mm jack (I/O change)
4, 4S, 5, 5S: Chamfered, flat, aluminum and glass design
4->5 Changes: make the back aluminum (material change), thunderbolt (I/O change), and make it overall thinner
6, 6S, 7, 7S (so far): rounded again, aluminum back, rounded glass front.
6->7 Changes? (from MacRumors): unspecified waterproof material (material change), remove 3.5mm jack (I/O change)
It seems to me that there are 4-year design categories with 2-year sets of devices. I would still expect to see a new design, but in the ilk of a 4->5 change or a 2g->3g design change. Still rounded edges, but with some material change (like they seem to do).