Run generic OS images
This is a "nice to have" property of the phone that I think is going to be necessary to have a vibrant ecosystem in the future.
In short, there should exist a standard of packaging the base OS so that any distribution could create an image which would then be runnable on many different devices. Such a standard is called EBBR, developed by ARM, and as of 2019-04, it's getting close to version 1.0. The end goal is something like the standard x86 behaviour where it's possible to plug any image onto any computer and it will work.