The chances of Belgium splitting any time soon are actually extremely low. You need to know the intricacies of the Belgian system and the feelings of the population to understand that. Only a minority of Flemings (perhaps 30% of them) want independence, and none want to join the Netherlands. If matters changed in the future, and Belgium did split in 10, 20 or 30 years from now (much more likely), Flanders would be on its own, but there is a good chance that Wallonia would join France.
Although Walloons prefer a united Belgium than joining France, few would be completely opposed to becoming the 96th French département. French departments are numbered alphabetically, except Overeses Departments which are numbered from 971 to 976. France even left a space for Wallonia at #96 between #95 (Val d'Oise) and 971 (Guadeloupe). I don't think it is just a coincidence. Many French politicians have already announced that they would welcome Wallonia as part of France, and most French people have no objection.
The eternal problem is Brussels. An originally Flemish city turned Francophone, the seat of the EU and NATO, this bilingual federal state could become a sort of European Capital District, the EU's version of Washington DC. In any case neither the Flemings nor the Walloons will ever agree to let it go to the other side, which is basically why Belgium is not likely to split. Nobody can afford to lose Brussels financially. The Flemish claim that they are paying for the Walloons' social security, but at present the only region that is really giving its tax money away to the two others is Brussels.
Something that could work out is an independent Flanders, and an independent Wallonia-Brussels state. If it all came down to a democratic vote, French speaking Belgians would choose this alternative before joining France or letting Brussels on its own. Ironically it is Flanders that would never accept an independent Wallonia-Brussels.