At the beginning of the 20th century there was the widespread belief the century will belong to increased liberalism and progress. All of Europe had some form of democracy, and even Tsar Nicholas was granting the Duma more powers. Reforms were underway in the Ottoman Empire and revolutions were toppling even the most stubborn dictatorships. Instead the 20th century saw the bloodiest and most insane totalitarian regimes in himan history.
So your challenge is to prevent such ideological totalitarian regimes from emerging. So no fascism, communism, or anything like that. There will inevitably be brutal dictatorships, but they must not be backed by a revolutionary and totalitarian ideology. There will be new ideologies, but they must not devolve into totalitarian regimes.
Bonus, people like Mussolini, Hitler, Stalin, and Mao are heroes of liberalism and democracy.