On the latest Librem 5 update, If you follow some links about plasma mobile, (what is and roadmap, mentioning used API like Wayland, Ofono). And especially the vizzzion blog posts on KDE, it almost becomes clear that this will be the foundation for the Librem 5, although Purism will not yet make that decision official.
I’m thrilled how complete it is already (see the video from five months ago). Here it is mentioned, that even idle power consumption is in a pretty good state, as it always has been a development goal. That is music to my ears 
Also, convergence is not a new concept to plasma mobile. It is already in the works, utilizing Kirigami.
To me it seems like the APIs are mostly set, what is missing is probably some integrational APIs, so you don’t have “walled gardens” between KDE and Gnome. For example, a common interface to the address book, as @mdt has suggested in a different thread. (Actually I have been waiting for this for almost two decades now, so that Kontacts and Thunderbird could use the same backend… well, it’s about time )