Adds heft and bulk. I doubt that Apple will go in that direction anytime soon. Wireless charging is slow and inefficient, which goes against where Apple seems to be leaning in both their engineering and green initiatives.
If anything, I see Apple dialing in big efficiency and battery life gains with the 6s and the 7. The 6s is already rumored for a die shrink, which Apple can use for either a performance or battery life bump, or a combination of the two. And since the A8 processor did not make any big changes to the microarchitecture, a design change with the A9 would also potentially bump up the efficiency and/or performance.
With the iPhone 6/6 Plus, Apple applied the efficiency gains of the A8 towards making the devices thin and light, and accommodating the larger screen and higher resolution. With the 6s, if Apple chooses not to go for huge benchmark performance gains, then they can raise the bar for battery life without bulking up the devices. And that would reduce the utility of wireless charging.