If it was up to me, I would use these basic start dates:
1821 - The Concert of Europe
1836/1837 - The Queen's Coronation
1848 - Springtime of Nations
1861 - A House Divided
1871 - Franco Prussian War
1884 - Scramble for Africa
1900/1901 - End of an Era
1914 - The Great War/The War to End All Wars
1919 - The Guns Fall Silent
I would also optionally suggest:
1815 (just after 2nd Treaty of Paris, Spanish Empire still alive, rest not too different from 1836)
1830 (July Revolution in France, last of the powerful Bourbons, instability in France)
1839 (First Opium War, collapse of Chinese Empire starts)
1850 (Taiping Rebellion, utter chaos in China)
1853 (Crimean War, first major war in Europe since Napoleonic Wars, Ottoman weakness starts having effects)
1857 (Sepoy Rebellion, last chance for the Mughal Empire and independent kingdoms of India)
1868 (Boshin War, Japan westernizes after one of the finest examples of gunboat diplomacy, Shogunate overthrown after around 700 years)
1898 (Boxer Rebellion, Europeans ransack China)
1904 (Russo-Japanese War, Japan defeats a European power for the first time)
1910 (Italian conquest of Libya, colonialism at it's height)
1912 (Xinhai Revolution, Chinese Empire ends after 2,133 years of glory, ROC proclaimed)
1923 (borders stabilize, world economy collapses, Fascists rise, potential for revolutions all over the earth).
But overall, in Vicky3 I think they should add dynamic start dates like in EU3/EU4 and CK2. This era was far more well documented with perfectly accurate dates (sometimes even to the point of hours and minutes), they can create a dynamic slider for start dates like in those games so that player can start whenever he wants. They can just highlight the major points of the era like in the those games, while where he starts is up to the player. It won't be too much of a problem for them.
There already exists a thread in Vicky2 forums for dicussing this though, and it has some excellent suggestions.