Just for the sake of speculating, this could bring lots of fun!
Imagine..
Flanders as an independent nation-state or in some union with The Netherlands. It shouldn't change much in terms of the percentage of immigrants other than adding the stocks of each into one joint state. Never mind.
I'd suppose that Wallonia has many less immigrants. And it has various options ranging from independence (less viable) to being offered some status within France, even as if in a protectorate of some kind. Never mind.
The fun comes with Brussels. What are the percentage of Muslims in the Brussels region, at present? I would imagine that at least some of the Flemish population would look forward to move out of there, and into Flanders. And maybe the same for the Walloons in Brussels.
In the end the population of Brussels, the "heart of the European Union", would be made up of a minority of ultra-tolerant multiculturalists (European population + eurocrats) surrounded by a majority of ultra-intolerant Muslims, many of the type who demonstrate with banners of "Europe is the cancer, Islam is the answer" and "Behead those who say that Islam is a violent religion".
The EU turned into the major soap-opera in history! I'm going to buy pop-corn before the shops run out of it.
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'Dardanidae duri, quae uos a stirpe parentum
prima tulit tellus, eadem uos ubere laeto
accipiet reduces. Antiquam exquirite matrem:
hic domus Aeneae cunctis dominabitur oris,
et nati natorum, et qui nascentur ab illis.'
We can easily forgive a child who is afraid of the dark; the real tragedy of life is when men are afraid of the light.
–Plato–
'Many people, I believe, wish for a society where faith, decency, pro-life convictions and national self-determination within Europe can flourish; and not be swallowed up in a dictatorial EU bureaucracy.'
–Gerry McGeough, Irish Nationalist and POW–