Prior to the Revolution, Brittany was not legally part of France, but considered a separate kingdom ruled in personal union. Say this arrangement is restored under the Congress of Vienna, and that following the February Revolution, Louis abdicates as King of France, but not of Brittany. This assumes that the Breton people still support him, as they did in that region's numerous royalist revolts during the revolutionary era. Is it realistic that the new French government lets them go, or would war and foreign support of Brittany (from Britain nervous about a new strong republican France?) be needed? And if Brittany does manage to split, what are the consequences? Does this weaken France enough to change the fate of the Roman Republic in 1849? Shift the balance of the Crimean War? Prevent the Entente between France and Britain after 1870, with Britain allying with Brittany instead? I think a lot of interesting things could happen here.