The challenge is to establish an alternate Belgium that while losing Flanders to the Netherlands, still manages to have a sizable population of Dutch / Flemish speakers and even a significant number of English speakers (who largely outnumber of the OTL German speakers).
The latter would entail the current OTL French departments of Pas-de-Calais and Nord (or at least large areas) eventually becoming part of this ATL Belgium, since the region was said to have once had a large number of Dutch / Flemish speakers.
It would also entail a wanked ATL Pale of Calais where the English likely engage in a program of Anglicisation (along with immigration of English speakers), prior to somehow becoming part of the Spanish Netherlands instead of the Kingdom of France as in OTL and eventually becoming part of ATL Belgium.
How would things in this ATL Belgium be different compared to OTL with the Walloons making up a small majority followed in descending order by the Flemish, English and Germans? Also which city would end up being the ATL capital city of Belgium in the absence of Brussels?