To make use of this combined playing field, it is necessary that the three games from the series are purchased in the same digital store. That probably means that this joint map won’t come out until after the release of the new Total War game.
The developer has not yet given further details and says it is now focusing on Total War: Warhammer III. This will be a huge folder that will be available for free as a dlc for these owners. Deloper Creative Assembly says work is still underway on a combined playing field for players who own the three Warhammer games in the Total Wars series.