As a side note to that, as much as it grieves me: Since the addition of Mark, the Westphalia duchy has become a bit too powerful imo, as it is now also a 6-province duchy. This is not that much of an issue, but gameplay-wise makes the area a better choice for say an imperial homeland than it historically should be.
A possibility would be the introduction of a duchy called "Engern" (Angria), which in the early days existed. But that's not a strict suggestion, just something to ponder.
There were historically quite a few duchies in the area another possibility would be the Duchy of Berg which existed from 1380 to 1806 when it became a grand duchy during the Napoleonic wars. It also ruled over Mark, Kleve and Jülich for some time.