Im only sending an idea here, one me and my player are talking about for a while.
We think the Old World is really under detailed in the current edition. It would be awesome to offer a fully detailled Old World setting guide with poster map as the 4th Hardcover book of this edition. In addition, they could provide a small vault products for those components complementing the guide.
The point would be to get a massive tome about the lore of the warhammer fantasy world. Could be even written as fluff only! This could incorporate Halfling, new monsters, new careers for neighboring realm to the empire (Bretonnia, Kislev).
What do you think?