The main problems of founding such a state would be:
1) The presence of non-slavic populations, such as the balts, hungarians, romanians, etc.
2) Lack of unifying factors within the slavic nations - different religions, languages, customs, politics, etc.
3) Great opposition to such a state by foreign powers (Germany, Turkey, maybe Britain)
4) Internal nationalism, particulary the Russian one.
Number one could possibly be solved by granting those nations the same rights as the Slavic nations would have (I don't suppose such an Empire would be a unitarian one), or by a Middle ages PoD preventing their settlement in Eastern Europe.
Number two would require an early foundation of such an empire before the rise of nationalism and the creation of a sort-of federal state from the various nations, allowing their populations to develop same political rights and customs.
Number 3 would have to be overcome by force.
Number 4 would require again require not only an early creation of a Slavic Empire, but also the termination of Russian chauvinism and their love for absolute rule. OTL pan-slavism became a synonim for Russian Imperialism and russification which the slavs themselves opposed. Thus russian power would either have to be reduced, or Russia would have to be excluded.
The best chances of creating such an empire would be either a more successful Great Moravia or other West-Slavic union (unifying the western Slavs, expanding southwards before the arrival of the Turks, and later east before russian unification) or PLC (forming a Polish-Lithuanian-Muscovian Commonwealth and expanding southwards by ousting the Turks).
There were a couple instances when the western slavs came close to be ruled by the same power. Boleslaus I briefly held both Bohemia and Slovakia during his rule. Vaclav II and Vaclav III managed to claim the title of King of Poland and parts of Lesser Poland. The Jagiellonians also held the thrones of Poland, Hungary, Bohemia and Lithuania in the same time under Vladislaus II, John I and Alexander I. Having any of them be more successful would be a good start.
As for a PLMC - there is the obvious Time of Troubles PoD with Vladislaus IV becoming Tzar of Russia. Alternatly, a Polish-Novogorod alliance under Casimir IV could extend into a personal union, and later a real union - this would limit Muscovian-Russian power as the Rus would not be single united entity within a future Slavic Empire.