In the 867 start, there should be no dutch culture. I'm dutch, but I'm going to be honest with my history books here. There were no people what we would call Dutch today.
Most of the dutch living below the dutch rhein would be called franks (or lower franconian speakers). These are "German Franks" to be precise and not the "Romance Franks" who live in france proper (paris). All the dutch culture we see should technicaly be renamed "Franconian", and that culture should be larger than is on the map right now. It should contain a fair share of the north of france and the rhein areas in germany (köln) for example.
Now the north of the netherlands above the rhein, should be frisian. And technicaly the area should be filled with little wealthy trading posts and fishing towns...(who all dissapeared when the oceans flooded the lands completely between 800-1200, decimating frisian population and areas).
In the process of the next 400 years starting from aprox 800, german immigrants from Saxony and the Ruhr area in germany started moving enmasse to the netherlands to develop the mirky marshes as endentured servants (as a grant by the emperors of germany). However many germans called "duutsen" by the locals would simply escape their lords lands and set up their own independent free duutsch towns in the protected marshes. (the modern dutch gain their name from these german immigrants, and it is also why were are not the same as flemish who are native.)
So here the north gets mixed Frisian, Franconian and a bit of Saxon culture.
Flanders remained largely franconian due to little immigration but was later influenced heavily to the south from the Romance Franks, turning half of belgium into Wallonians and the north of france into Frankish Romance speakers from Franconian.
Flanders should go into France, it was historicaly most influenced by it and part of West Francia. The Netherlands should go to the HRE, because thats who influenced it the most (and Frisia had more in common with saxons than with French). And even one of our counts became emperor whuuhuu:happy:
Edit:
Frisia as a kingdom?
Ludicrious!
They shouldn't even have provincial status in our own national administration