Yeah the main attraction a Surface phone would have would be the ability to run Win32 apps. If it's just another Arm device then it's just another Windows Phone, it sadly doesn't matter how good it is.
So that leaves some kind of phone which likely transforms into a PC perhaps a hybrid Arm/x86 system with shared storage but x86 CPU/GPU in dock connecting via Thunderbolt 3 than allows you to run Win32 apps but when in Phone mode it can only run UWP touch apps.
The dock approach would be ideal as it could be used an upgrade system so your "PC" isn't left obsolete, even do a games focused version as proper AAA games is one of the few things Windows has going for it and something iOS/Android covet but cannot replicate yet.
Also loosen up the Windows Store, support more ports of Win32 apps, the large semi-hostile to MS PC user base still see the Win store as a mobile garbage dump, until proper apps start filling in it's ranks like AutoCAD, WinRAR, Refect Backup, Steam, Malware-Bytes etc no-one is going to take it seriously.
If it's just an Intel Atom in a phone running Win10 with no means of expansion then it's got no chance outside of a few business customers.