Has there been any interest in bringing the wonderful alternative campaign settings (Saxons!, Pagan Shore, Savage Mountains, etc.) into the 5th edition and the GPC, possibly incorporated into the Warlord/Estate/Uther system as well? I know that the Book of Knights and Ladies goes some part of the way in accomplishing similar things, but it's nothing to compare with full books for lore, culture, geography, and politics.

These are, at best, part of a long-range plan.
Several other things have priority at this time and are actually closer to being finished, such as Book of Sires, Book of the Castle, and Book of Magic.
No schedule yet for those, but they are nicely in the works.
We'll announce what is available when it is.

Happy to see Castle is coming...I've been waiting so much for that.
Regarding the rest: I feel that you MAYBE mean that the regional books will appear (if they appear) not earlier than in 4-5 years...truth to be said I feel that we need something like that for Logres too and not just for Cambria, Cumbria, etc...
For a newbie GM/ Player it is frankly a mess trying to coordinate info from the GPC (using old names) and info from Warlord/Uther (using new names).
The county names in the Book of knights are senseless without description and some cultural data.
We need a kind of Atlas with a Gazetter of all Logres sites and lands.. .

I am in agreement Luca, and these are being addressed.