I too would enjoy a 5e WotC series of region supplements though. I enjoyed them in the AD&D era and I enjoy Paizo's versions of them for Golarion.
The green book series was one of my favorite for AD&D. Made good use of A Mighty Fortress and Glory of Rome (they had quite a number of others too).
The setting material at the time I think benefited a lot from the boxed set format. They could put so many little things in those to help bring settings to life. I don’t imagine we will see that kind of boxed set as the norm again anytime soon but WOTC is in a position I think they could feasibly make a return to that approach