Ok, so this is my (rather ASB) idea...
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Basically, the Belgian people, and more importantly the Belgian parliament, get pissed at Leopold for his antics in the Congo, and the parliament delivers an ultimatum: Turn the Free State over to direct state control, or we're removing you as King.
Leopold tries to resist, but ultimately, it turns out that the soldiers can only shoot into the unarmed masses so many times before the army breaks down. Leopold is forced to flee, and soon many royalists and elements of the army follow, a new "Kingdom" is declared, although as the royal family is forced into exile, its more or less of a Regency. Britain and France, eager to remedy the embarassing PR situation involving the Congo, recognize the new government, while the Germans, along with most of Europe, remain insistent that the rightful government be restored...
Meanwhile, finding themselves mostly pariahs, the Royalist faction heads for the Free State, and soon settles itself in, declaring Leopoldsville as the capital of the "Kingdom of Belgium-Congo". The Force Publique, not eager to be tried for "crimes against the subjects of Belgium", agrees to support the new kingdom. Leopold, now finding himself in control of a few thousand new subjects, agrees to set up a new system of "reclamation" in the Congo. The country is essentially feudalized, as the Force Publique are unleashed as a new sort of warrior-caste on the native peoples, clearing whole swathes of the country of any human life, and resettling it with new, white-ruled territories given grandiose titles of royalty. Those native Africans that survive are either turned into slave labor (Slavery is put into law as the "performance of racial duties") or simply exterminated as being a waste of resources. Naturally the death tolls in regard to the natives is high, and the new settlers suffer both from tropical diseases and conflict with rebellious tribes.
The Great War breaks out eventually, although this time, the Germans decide to rectify the situation in Europe, and invade Belgium with the intent of "restoring the rightful government of King Leopold". The war has some major changes, the largest being a German victory in Europe and an Allied victory elsewhere. Belgium-Congo ultimately enters the peace conferences as a major victor. Leopold is restored as the rightful king in Brussels (in exchange for some "territorial adjustments" in the favor of the Germans), and even grabs some pieces of French territory as "compensation" for their support of the Regency-era government. In Africa, the British, eager to both continue to have a trade-partner in Europe and in "gratitude" for Leopold's neutrality, gift to him Ruanda and Urundi from the Germans.
Post-war Belgium-Congo (Leopold decides to keep the "kingdoms" separate in order to maintain his personal dominion over the latter) is a bloody place, as the Force Publique perpetrates the same ethnic cleansing in the new African territories, while in Europe, Leopold's iron fist unleashes a new "White Terror" in Belgium which sees hundreds of political opponents and Regency-supporters being executed and tortured. An absolute monarchy, backed by German hegemony, British commerce, and African wealth, Leopold's domains are among the bloodiest in all human history...
