Wallonia doesn't have a large enough population, territory or economy to make much difference.
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A reunited Netherlands would be like a Poland in the west. Population of the country would be increased to 28 million, its GDP increased to $1.3 trillion, its territorial reach increased to something-90,000 km. Another modest gain could also be made if Luxembourg were to become part of this reunited kingdom.
http://www.zum.de/whkmla/histatlas/l...s/nl181539.gif
What is being called 'Flemish'-speaking on that map is actually Dutch-speaking. To me Flemish is only the historical South-Western dialects of Dutch, all other claims are anti-intellectual.
Sadly very prominent in the 19th century. All Germanic-speakers were collectively called 'Flemish'-speaking, hence you even see the German speaking parts being called such in the map above. The yokels hijacked civilisation in that era. Be aware that we're talking about Belgium here. :p