Since the plan is to aim to reuse as much of existing XPages apps in new apps as possible, it would be tremendously useful to make the parts that make up the XPages stack as available as possible.
The ideal would be open-sourcing it entirely. Second to that, though, would be making the jars available both as a p2 repository and as Maven artifacts, in both cases ideally publicly (or near-publicly available). This would make the process of developing XPages plugins significantly easier (the "Domino Update Site for Build Management" project on OpenNTF was a good start, but it's very out of date) and would also make it much easier to develop tools to help either make REST-service "XPages" apps easier to create and to drag XPages-locked code out into another app-dev environment.
Since agreeing to licenses may be an issue if the stack isn't open-sourced, OpenNTF is willing and able to host Maven and p2 artifacts in a repository that requires specific authorization to use, perhaps contingent on agreeing to the same kind of license as in the Community Edition.
Unlike with https://domino.ideas.aha.io/ideas/DDXP-I-139 , the idea would not be to re-use the code directly in JSF, but to aid the community in creating tools to help portable development.