In order of importance:
1: Large display, or, even better, virtual side bezels (upper and lower bezel would stay, side bezels would only appear in tablet mode; when attached to the keyboard, they would make the screen 16:9, so larger and no more bars in videos, best of both worlds)
2. Core M with 8GB RAM and 256GB SSD and option for core M7 (current Skylake core M7 has the same power as the i5 in SP3). Like HP and Lenovo are already doing....
3: Second USB type C (3.1 second gen), in addition to the current 3.0
4: LTE option
5: power type cover
6: bigger battery
What I wouldn't like:
1. stop trying to make it thinner and reduce battery size, it's thin enough (lighter ok if possible but without compromising battery)
2. stop increasing the resolution, it's more than sharp enough and this creates issues with battery like and scaling.
What I would like but it's not gonna happen:
silo for a small but not too short second pen (like the amazing toshiba Z20t-B-10C)