I doubt that Belgium will still exist in 2035, although I cannot see the country splitting before another decade from now.
Many scenarios are possible. Flanders will most likely want to be on its own rather than with the Dutch, with whom they are quite culturally distant, language aside.
It gets more complicated for the French and German speaking parts. At present the politicians seem to favour an alliance between Wallonia and Brussels, hence the creation of the new Wallonia-Brussels Federation. But some Brusselers would prefer to see Brussels as a European Capital District, like Washington DC. In that event, Wallonia could stay on its own (unlikely) or join France.
The small German-speaking community of Wallonia would then have the choice to join France too (after all Alsace and Lorraine have their German-speaking communities too), rejoin Germany, or join Luxembourg, what I hear is the most popular alternative because of their French-German bilingualism, and because both areas speak Franconian German dialects rather than the 'new' standardised High German.