For me, the integration of W10 is a big draw. I just bought a new laptop with 8.1 on, and it has dragged so many bits over that I am hugely impressed. My passwords, my favourites etc. W10 universal apps are a great idea.
Then you add the flagship phone in to the mix, and I will be a happy shopper. I'll be honest, I am setting my sights in a sensible range, so I shouldn't be disappointed. Qi wireless charging is an essential. A 1080 screen, with 2 or 3 GB RAM. An 810 Snapdragon. inside. 20MP camera with a top class sensor. Front at 5MP with a wide angle. A 5.1 inch screen and a 2800 battery. Capacitive buttons, not on screen. As you can see, they don't have to go much beyond what they already offer in other phones (735,930,1520).
To be fair, it is difficult for me to believe they will offer less than the above specs. If they want to exceed expectations, then they can add:
a bigger battery
3GB RAM
Snapdragon 820
Some fancy Iris scan.