Five Nations was a 160-page book covering Aundair, Breland, Cyre/Mournlands, Karrnath, and Thrane. That's about 32 pages per nation – Cyre/Mournlands got a bit less but that was used for the non-specific parts of the book – and that seems about right to me. So an updated version of that and two more for the other nations – could do one for the east and one for the west.Source books? That would be only one book with with about 8 to 10 pages a piece. It think that's what WotC for them back in 3e, I think. (Explorer's Edition?)
Now, I don't expect them to do that, because capitalism. But I don't think a one-and-done setting book counts as support. Support requires an ongoing commitment.