Essentially, from what I can make out of it, the Congress of Vienna (called the Dancing Congress) saw ministers for the great powers - Metternich, Talleyrand, Castlereagh etc - "shaking the map free of pins" (as the one biography on Napoleon called it).
Many post-1815 governments tried to set the clock back to 1789 - the "Holy" League chief amongst them.
What if, due to the 100 Days, or some other event, they decide to accept that the French Revolution happened, the reforms Nappy put in place are not so bad (some of them, anyway) and build on those reforms?
Josef II, Leopold II, Ekaterina II and several other Enlightened Despots had started a liberalizing trend that the Revolution scared them out of. What if in 1815, those applicable get dusted off and re-examined?