Honestly, that is a much larger question, which I cannot answer. But an unlockable bootloader would help as a first step — preferably one where usage of which doesn't invalidate the warranty. Requesting this via twitter from @sarvikas (Chief Product Officer of HMD Global) and @MikkoLJaakkola (CTO of HMD Global) is one course of action.

Another would be simply not purchasing any of their devices until they implement this.

Another might be a public petition to HMD Global.

Another route would be to ask GSMArena and all gadget review websites to make it very clear whether a device includes an unlockable bootloader or not, and whether doing so invalidates the warranty. This would awaken the mainstream consumers to the the feature as being important to look for.

Really, for your main question, you would need to ask Jolla what is required.

I think that is partly why Jolla chose this phone first: availability, cheapness, openness, unlockability, and 'documentation'. I think there is definite demand for an officially supported Sailfish release on a high end handset. My guess is that will be another Xperia (though some people claim the camera/display are still not high-end enough on the current premium models).