What about an ATL where Britain retains Florida, instead of being forced to cede it to Spain in the 1783 Treaty of Versailles? As such, the British effectively acknowledge the Seminoles', especially the Black Seminoles', independent sovereignty by either providing them First Nation status or making their state a British protectorate. The Seminole State, Maroon Florida, becomes a critical buffer state for the British against the U.S of Americans who want to possess Florida themselves- and the US slaveholders become increasingly aggressive against the Seminole State themselves when they seek the capture and return of Florida's black fugitives, set out under the Treaty of New York (1790). ITTL's War of 1812, The Maroon Seminole Nation in Northern Florida is already a critical ally of the British in the region, with a military force which has already been overtly supported, supplied and trained by the British for decades, and joins the British in their war against the United States.
The Seminole State's competent, decently equipped and far larger armies are extremely successful in their battles against the Americans, seizing significant swathes of territory in Southern Alabama and Georgia (and retaining them after the conflict); and their agitators, sent behind enemy lines, are even more successful. Soon, much of the USA's slave population on the Southern plantations is in open revolt, and the USA is forced to begrudgingly sign a premature Emancipation Act to quell the uprisings, and to stem the resulting flow of willing recruits swelling the Seminole's armies and boosting their supply chain.
After its conclusion though, the American population is far, far more hostile towards its newly freed population of former black slaves, who are popularly portrayed as traitors, with America's defeat in the War of 1812 directly attributed to them (in spite of the fact that large numbers of those slaves joined the American armies, and fought against the British and the Seminoles for their freedom prior to TTL's Emancipation Act). As such, a mass migration ensues, with the freed Black peoples of the USA fleeing en-masse to the expanded Seminole State and the British Dominion of Florida, and virtually all of those black people who attempt to stay behind in the USA subsequently killed in the retributive genocide.