In my humble opinion, the iPhone 7 is essentially an evolved version of the iPhone 6 design (just like the iPhone 5 was an evolved version of the iPhone 4 design), but adds the following features:
1. No more Home button. There will be a "virtual" Home button on the bottom center of the touchscreen, and the bottom fifth of the touchscreen will become one big Touch ID fingerprint sensor area.
2. The new A10 system on a chip (SoC) will still be dual-core but with a lot more generous cache RAM and more efficient CPU internals. RAM memory will now be 4 GB standard.
3. Touchscreen may go to sapphire glass for the first time, unless Apple uses something like the successor to Corning's Gorilla Glass 4.
4. Resolution of touchscreen will not change compared to the iPhone 6 or 6+.
5. 3D Touch may get some minor enhancements compared to now.
6. Internal storage will be available 32, 64 and 128 GB versions.
7. Front image sensor may sport slightly higher resolution; rear image sensor will have its lens element flush against back of the case, with LinX dual-lens rear sensors on both the iPhone 7 and 7+ models.
8. Improved Lightning connector for sure, possibly with USB 3.0 connection speeds.
9. May or may not keep the 3.5 mm headphone connector (this will likely be decided by the end of February 2016).
In short, at first glance it may not look much different than the iPhone 6, but closer inspection reveals some major hardware changes.