For someone from Groningen it's offensive to be called a Frisian. So you will nearly always have problems with that. I agree that it's better if those territory's were not named after city's though.
Historically Groningen was part of frisian...
I suggest to keep 2 territory's and name then Northern Netherlands and Southern Netherlands, but if every other territory is named after a big city within that region then I would keep those names (Amsterdam, Rotterdam).
Both Northern Netherlands/Southern Netherlands or Frisia/Brabant would be fine with me but not cities because Belgium is called flanders and wallonia.
Edited 6/2/2015 22:12:10