As much as I hate to say it, Surface Phone is best to wait another year. The "retrenchment" year has killed Windows Phone, and remember all the early talk of Redstone2 was that it would be focused on mobile? Now it's focused on 3D creative for the desktop.
With all of Windows Phone's momentum having been lost, and the mobile market saturated with a sea of iPhone clones, Surface Phone needs to bring OS innovation to the table the way that WP8 did when it was fresh, exciting, and disruptive.
At minimum, we finally need interactive live tiles, a return of hubs, full potential of the People Hub, and deep social media integration again. I know it sounds like a rehash of "old features", but think of what we primarily use our phones for. Forget the "app gap", when I think of it, I hate apps, lol. The social features at an OS level were absolutely brilliant. I love browsing the People Hub for "What's New" on WP8 in the Metro Dark theme, rather than having to load up the facebook or twitter apps.
Mobile hardware has hit a plateau. Every phone has a top quality display, the days of Lumia Pureview being a cut above the rest is long gone. Any modern phone has a great camera. OS innovation is the next step, although I'd love for Surface Phone to introduce some stand out hardware design, the way the Lumia 920 / 1520 did. I don't want a generic looking "iPhone clone" phone.
Surface Phone needs to be about a next generation user experience as a communications / connected device. Something to make us look back at the "there's an app for that" era and laugh.