Probably not make any difference. Flanders would go to Holland and Wallonia to France and perhaps part to Germany and Luxembourg as well. The EU headquarters would suddenly be in Holland however.
“Flanders would go to Holland and Wallonia to France and perhaps part to Germany and Luxembourg as well.”
As a practical matter, Dutch and Flemish are the same language (they’re indistinguishable on paper, though the local dialects and accents vary), so you’d think merging Flanders and Holland would be easy. It would make more sense than the present jury-rigged arrangement of French and Dutch-speaking regions pretending to be one happy country. From what people who live there have told me, however, the Flemish nationalists want their own country and do not want to join Holland. There are local resentments, grudges, and religious differences, not to mention historic reasons (Belgium came about as a revolt for independence from Holland in the 1830s in the first place). And Holland has enough trouble balancing its own array of political and religious factions to want to absorb millions of new voters. Supposedly, studies have shown that Flanders has the economic potential for sustaining itself as an independent nation. Wallonia, on the other hand, is poorer and landlocked, and union with France might be its best option.
From what I've read on the topic, I would have to express sympathies with the Flemish.
They appear to be the more industrious of the two groups, but consistently get outvoted by the French speaking majority. There comes a point when they simply can't take it anymore.