As of yet, I'm most likely adding 4 provinces, being Upper-Guelders, Upper-Sticht, Limburg and Namur. Most likely using the Rhine Weser Oder and an as of yet undecided 4th river. I now know what you meant by saying 'which would have required doing a considerable section of Germany as well' God the Rhine is BIG!
For anyone interested; Work in progress, I'm about halfway with province IDs:
I feel bad for butchering Germany like that

Might add in Jülich in the north corner of Andernach in the end, not really decided about that yet.
I don't like this set-up for the Northern Low Countries very much, it looks more like a map for the political situation at the start of EUIII then that of the 11th century.
Sticht should be bigger ('t Gooi and that Holland part south of Sticht) should belong to Sticht, Holland didn't conquer those area's then very late in the CK-era.
I think Guelders should just be named Gelre (all Dutch provinces have Dutch names, why make an exception here).
Almost all christian provinces on the map represent some independent political entity (either a lordship, country, bishopric and so on) neither Upper-Sticht or Upper-Guelders fall under that. Upper-Guelders was Gelre/Geldern. At some point the rulers of Gelre/Geldern also become the rulers of a number of small counties/pagus (Veluwe, Betuwe) in what on your map is called Guelders. The area then became known as Gelre
My suggestion would be to remove Upper-Guelders and keep Jülich there, which was a historical county (with its own ruler) in that area.
Now you probably thought that I was joking about including the county of Zutphen, but I wasn't
The county of Zutphen existed from 1046 to about 1590 (the last count of Zutphen was king Phillipe II von Habsburg of Spain). Untill the mid 12th century the county had its own ruler and after that it was in a personal union with the count of Gelre (Gelre became a duchy in the 14th century).
In the mid 11th century Zutphen was for a while even the residence of Heinrich III (the Holy Roman Emperor). In 1066 the count of Zutphen was Otto II of Zutphen, his uncle (the previous count) Koenraad/Conrad had also been the duke of Bavaria, but was disposed after a quarrel with the emperor. Koenraad/Conrad was related to the Ottonian emperors (the dynastie that ruled the HRE before the Salians/von Franken).
Otto II was closely related to another powerfull German family (the Ezzonen, von Bonngau in the game), who were the heriditiary Counts Palantine of the Rhine. Otto II married Judith von Supplinburg, the sister of Lothar von Supplinburg, who was Holy Roman Emperor from 1125 to 1137. His eldest son, Diederik/Dietrich became the bishop of Münster.
In other word he was a man with powerfull connections in Northwestern Europe. And since the county of Zutphen was his main title, it should be obvious that it was also an important county.