Personally, I think it should be much like the 3e FRCS (but updated). Update some of the subclasses and backgrounds from SCAG, add some new ones. Put in some monsters in, maybe some feats, and some magic items. Give a page each of info on the other continents.
I do think they will use the FRCS as a template and a starting point idea of what FR fans want from a setting book, but as I said I think it will have lineages as well, maybe alot of them. It could bigger too.