WW1 seems exceedingly early for a Belgian break up, except perhaps in the case of a German victory - and even the Germans during the period of the
Flamenpolitik were only lukewarm supporters of Flemish autonomism, even annulling the decision of the collaborationist Council of Flanders to declare Flanders' autonomy. By that time, the large majority of the Flemish movement still hoped for a realisation of its goals within a unitary Belgium - indeed, the complete bilingualism of Belgium remained the goal of most for a couple of more decades. And Walloon nationalism was still in its embryonic stage, still dominated by groups (often within Flanders, hence 'the false Walloon movement') hoping for a complete frenchification of Belgium rather than Walloon autonomy and independence.
The most likely case for a Belgian break during the time when Congo was still a Belgian colony, seems to be around 1950, if the Royal Question intensifies and leads up to Walloon separatism and the plan for a
provisional Walloon government (lead by Joseph Merlot?) is put into action. It would still be a far shot, since I doubt that the US and the other western powers would welcome a left wing separatist government, despite the apparent belief of the separatists that they would be welcomed by France and the UK.
In that case, I could see Congo becoming a Flemish colony for a certain amount of time, as strange as it might seem today. Or rather: it could see it remain subjected to a Belgium, reduced (and perhaps renamed) to Flanders (including Brussels). After all, Wallonia separated, why should it take the colonies with in? Furthermore, conservative and clerical Flanders was, at that time, the stronghold of the monarchy and the catholic church, two institutions often credited with more power in Congo than the Ministry of Colonial Affairs.
However, I could also see these events as leading to an earlier intensification of Congolese nationalism, and thus an earlier Congolese indepencence.