Before WWI, create a scenario where Russia is able to defeat the Turks and force them to cede Thrace, Constantinople,as well as the Dardanelles and Bosporus straights. Whether the Turks and Greeks flee is up to you, but the Russian's should heavily colonize their new territory becoming the majority by the outbreak of hostilities in an European colonial war (roughly around WWI).
How that plays is up to you, but lets keep the Revolution in Russia, with Constantinople becoming the new home of Tsar Nicholas II and his family.