Spa are apparently facing future financial troubles at holding the race.
So why not alternate between the French and Belgian Grand Prix, but keep it as Spa? One year it could be subsidised by Wallonia, and the next by France. It’s better for France than having no GP, which is how they find themselves currently (and against their will, it should be pointed out).
From speaking to Belgians it seems unlikely to me that Belgium will actually split, though if it did the constituent parts would almost certainly join other countries, leaving Brussels as the DC of the EU. But I think the idea of sharing the track between two countries could work a treat.