There was some talking in the OXCE+ community to unhardcode relations between the three factions (X-Com, alien and civilians), and possibly add more factions, but it's a complex thing. At least the idea was already proposed, so we'll see.
I think the factions are VERY well defined at the moment.
Most of what alinare writes -- depending on exact interpretation -- is already doable now.
The discussion from Solar's wishlist to "unhardcode" and add more factions is quite vague and blurry; definitely needs more thought before it can be taken seriously.
And regardless of that, code-wise this would be a major breaking change, which is explicitly written as not being a goal of OXCE+.
So, before any more discussion and brainstorming arises, I'd like to point out, that the current system of factions will stay unchanged in OXCE+ (most probably forever).
Cosmetic changes are acceptable, but have to fit well with the current system.