Its also a 64-bit device. I don't know what's the current status of 64-bit devices now among ported devices, but getting SFOS ported to this architecture is a major success and we should not underestimate it. When done, it should open many current devices for all of us through the ports. I do wonder if the port for Xperia X would be made available as a community port image / sources as well. That would allow for curious user to test SFOS before getting full version of it. If not, porters would probably know or would get help to get SFOS running on X Compact and other variants.
As for the pure ports, if you don't need Android stack, the current main problem is the absence of text prediction (jolla-xt9 is proprietary). So, would be great to get someone to work on it and get open-source solution for text predictions. Port from Ubuntu Touch, for example?
Check out OKboard if you haven't already - continuous input /predictive keyboard for Sailfish that is fully open. I can't remember whether it does predictions for normal typing (I think it currently uses xt9 for that) but it clearly could, because it does its own predictions for continuous input. That's something open versions of Android are missing BTW - if you want CI on Android you have to use something like Swype or Swiftkey.