Another alternative could be some kind of breakdown in the functioning of government shortly after the adoption of the Constitution. After all, if the republic fails here, it would be the second time republicanism fails America. If both Washington and Adams die before the Presidential Succession Act of 1792 is passed, then disputes over who is President would likely trigger political chaos at the bare minimum and while the USA could just splinter, a civil war over succession isn't too unreasonable either. Perhaps the chaos inspires politicians to consider crowning a monarch to serve as an "anchor" for the government. Alternatively, if the situation never calms until the rise of Napoleon, perhaps a Napoleon-esque monarchy forms in America, intending to forge a new, stable monarchy to replace the chaotic and corrupt republic just as Napoleon did.