Make "Russification" of the Finns Poles etc, "Germanification" of the Poles, and "Magyarization" of the Romanians and Slovaks more successful in the late 19th and early 20th centuries.
The Russian authorities never really hoped to transform the Poles into good Orthodox Ruthenes. The repressed Polish cultural and political activity at times when they thought it was a political menace to the empire, but the speaking of Polish in homes and villages and the Catholic faith were never really attacked, and it would be very, very difficult for such attacks to have any success. As DrakonFin points out, even the measures that were taken, in Poland as well as Finland, were counter-productive.
And in many other parts of the Russian Empire, de-Russification took place. In 1914, there were probably a greater proportion of Russians in the northest Caucasus than now (they were Terek Cossacks and got deCossackised), and all the big cities of Belarus and Ukraine were Great Russian (or Yiddish-speaking with Russian as the next biggest language and language of business), plus Vilnius. There were a much greater proportion of East Slavs (Russian and Ukrainian) in Besserabia.
Likewise, in the German partition Polish had been pretty much ruralised in 1914. In neither case do I see an obvious way to do more than what was done already.
In Hungary, have no 1848 revolution. It resulted in some very low-level tolerance (primary schools, mostly) for Slovak and other Hungarian minority languages by the military regime, and the creation of strong institutions for Serbs and Transylvanians. Though difficult to do in itself, no Czech revival harms Slovak a lot, whereas no creation of Serbia and Romania can only help.