No idea what they think about it themselves, but I (not a citizen of EU) would rather see Niederlanden out of this. In my classification Niederlanden is a more or less nice country (I'm talking about foreign politics, not weed), Belgium is shit and Luxembourg is small and I haven't heard about them anything. So the mix would inevitably be just shit.

The idea of the Netherlands and Belgium merging is completely detached from reality, adding Luxemburg to that picture makes even less sense. Economically it could be great with the combined harbors of Antwerp and Rotterdam. But historically and culturally it clashes, even between The Netherlands and the Dutch speaking part of Belgium.

I have seen too much cultural resentment toward the Dutch for this to be a viable scenario, both from the Flemish and the Wallonian side.

Also, both the Dutch and the Belgians are deeply racist. You can't unite racists. (Source: am Dutch)

De Wever is a far-right populist, this is just bluster.

Don't forget about the political clash as well. Flemish separatist movements have always ran into a brick wall: Brussels is majority French-speaking, entirely within Flemish territory, and wants nothing to do with their nonsense. No matter how much Flanders might want to cut off Wallonia and reunite with their orange daddy, they won't be able to bring Brussels with them, and that would be political and economic suicide.