When I played the 1813 campaign in noticed that Belgium and the Netherlands were still independent kingdoms. For the Netherlands I can understand it because between the kingdom of Holland (under Napoleons brother Louis-Napoleon) wasn't that much different from a French province.
But the kingdom of Belgium is completely out op place. That kingdom was an ideal that wasn't realized until 1830, it looks really strange that king Leopold is ruling between the two Bonapartes in France and the Netherlands. I don't know why it was chosen to place Belgium in the campaign but for LME which can pride itself on historical accuracy it seems strange.
I think in future updates Belgium should become part of the French empire or of the Netherlands to enhance the Kingdom of Hollands gameplay.