In the 20th century?? Umm.... the USSR and its 'empire' (the COMECON) was the closest thing to an empire. Other than that, I see no remotely possible way to have something like 100 million people (Bulgarians, Serbs, Croats, Poles, Czechs, Slovaks and others) just forget about nationalism and happily submit to rule from faraway Moscow.
The early 19th century might be a better time for something like that. Have the world go up in a major war between, say, England and France and the Ottoman Turks, have Russia invade the Balkans and win big time against the Turks while the other Great Powers are fighting a MAJOR war (like, Germany and Austria versus Denmark, Sweden, the Italian states and France, and England also involved both in this war and an all-out war on the North American continent)...
The thing is, under normal circumstances the Great Powers would have intervened whenever Russia got too powerful. See Crimean war, see 1878 conference of Berlin. Russia had finally managed to stomp the Turks, and had created an uber-Bulgaria mostly out of scratch in order to get a permanent foothold in the Balkans. But the other Great Powers wouldn't take it and made Russia and Bulgaria give up a large part of the conquered land.
Under the above mentioned circumstances, the Russians *might* have seized the opportunity to remodel much of the Balkans. Prerequisites would be a favorable situation in western Europe taking British, Austrian and French attention (not something small, rather like Napoleon runing wild again) and the Russians being able to _quickly_ break the Ottoman Empire apart and reshape the Balkans as they see fit. (Maybe also taking Constantinople - an old dream of the Russian Czars.) What you get when they're done is an expanded Russian empire (Romanian principalities annexed) plus a Balkan state that has been expanded to become Russia's ally of choice. Maybe Bulgaria (useful because close to the Turks, but maybe they'd be annexed as well - Bulgaria was an underdeveloped mess prior to the 1860's) or a greater Serbia that has grabbed Bosnia and Albania.
Eventually you might see the war in the west end, and Austria tries to halt the Russian advances on the Balkans and destroy the Russian ally on its borders - through diplomacy, and once an alliance has been secured, through war. This would be around the 1840's or 50's. Russia ought to get a few allies as well (maybe the losers of the previous great war, *g*) and when they're done smashing up Austria, they annex Galicia to the Polish kingdom (whose King the Czar would be if he hasn't already abolished Polish autonomy as he did in OTL), attack Bosnia to Serbia and perhaps break the catholic southern slavs (Croats, Slovenes) off Austria as well. But there's a large cultural divide... just like between Poles and Russians. Difficult. Oh, and Bohemia would need to be attended to as well... annexation? Union of crowns? Something like that. Anyways, _all_ of Russia's western neighbours would need to be in total disarray in order for this to work out.
Assuming the rest of the world _still_ doesn't gang up on the Russian Empire upon seeing how powerful they have become, and assuming the Empire doesn't all fall apart under Czarist mismanagement, by the end of the century Serbia and/or the other Russian proxies could have merged with the Russian Empire and you have an empire spanning from the Vistula to Vladivostok, and from the Golden Horn to the arctic circle.
This empire would be so polyglot and culturally diverse, and economically so heterogenous that democracy or constitutionalism could only be introduced in very, very small doses. Absolute monarchy would be a sensible way to rule all these lands, with not just one iron fist but probably two or three.
Title of the ruler: Czar of all Russias, King of Bohemia, Poland and Serbia, Grandduke of the Bulgars, Emperor of Constantinople (how does Emperor of Rome sound?), protector of all Slavs. Something like that.
Assuming nothing goes wrong, this Empire utterly dominates central Europe. (Mental note: huge army required) Nationalism in eastern Europe doesn't really happen, and when it does (1880s-1910) the Empire is slowly transformed into a constitutional monarchy with limited autonomy. Eventually, as the 20th century dawns, the internal strain reaches the bearable limit and the democratic structure collapses just as it did in Austria before 1914... the Duma (parliament) isn't summoned any more because the parties (ethnic parties for the most part, plus a smattering of liberals and socialists) cannot agree on anything except that they all want more autonomy, and instead the Czar's prime minister rules through decrees, and the army feels that the civilians have been running things their way for way too long. The common people feel that the enormous economic boom isn't benefitting them enough, and demand more changes as well. The Socialists fare alarmingly well in the sporadic elections.
Just when they start looking for a chance to turn people's attention away from the hysteric name-calling in the Duma, the czarevich, Grand Duke Franz Ferdin... eh, I mean Mikhail Aleksandrovich Romanov
, is assassinated by a Hungarian nationalist while on a visit to Klausengrad. (Formerly Klausenburg, or, in Romanian, Cluj-Napoca; capital of Siebenbürgen, which was annexed to the Empire following the collapse of the Habsburg Monarchy, and is regarded by Hungarian nationalists as rightfully theirs.) Czar Aleksander III, 69 years old and mortally sick, is in no shape to exert a moderate influence on his government at this point, and the government under Count Sturkovich issues Hungary with an ultimatum. Hungary complies, but the hawks in Moscow succeed and war is declared. Hungary's allies - the German Confederation under Prussian leadership, France, Britain and the Turkish republic - join the Hungarians, Sweden and Greece join the Russian side. The war to end all wars breaks loose, as all of Europe between the Adria and the Baltic Sea turns into a battlefield.
How does that sound?