So, historically, what countries do you think had the potential to become great powers/empires but remained weak and (relatively) insignificant on the world stage instead of becoming great empires? What PODs would have made said "empires which never were" develop and why? What of said "empires which never were" do you think were the most fascinating and why, then?
Of course, term "great empire" is very broad but how about a little bit different fate of the Great Princedom of Lithuania? By the time of Vitold (Vytautas the Great - mid-XIV early XV centuries) Lithuania expanded to include, in modern terms. Belorussia, most of Ukraine, and Western Russia (Smolensk region). Plus, Vitold was recognized as an overlord of Novgorodian Republic and "Protector" of the Princedom of Moscow (hold by his grandson under the regency of Vitold's daughter).
At some point there was a real possibility for him to go further: by an agreement with an exiled ruler of the Golden Horde, Totkhamish, upon restoration he would acknowledge himself as Vitold's vassal and pass to him the rights of the sovereignship over the Russian lands. The plan failed because on the opposite side Vitold had Edigu, founder of the Nogai Horde and presumably the last great Mongolian general. Defeat at Worskla River cost Vitold dearly, especially in the terms of his greater dependency from Poland. But Edigu was a "subjective factor": he could die prior to the battle or he could be brought on Vitold's side by a promise of making his horde an independent emirate. Vitold is victorious, his prestige is skyrocketing and he is an official supreme ruler of all Russian lands and of the Golden Horde's territories stretching to the Ural Mountains.
At that point he has few relatively simple things to do:
1st, arranging for an early demise of his brother, Sigismund Kęstutaitis, and his cousin Švitrigaila (if I missed some other male members of the family, kill them as well; of course, except for Jagello). That would be well within the scope of acceptable of this nice family.
2nd, to stick to his alliance with the Teutonic Order against Poland and stop meddling in Samogitia: this tribal area was not worthy of trouble and spoiled relations with the only force which could support Lithuanian independence from Poland.
3rd, while remaining a Catholic would be a prudent thing to maintain the good relations with Teutonic Order, take care that the Orthodox nobility (and population in general) on his lands have exactly the same rights as the Catholics.
4th, use his Teutonic allies to achieve recognition of his complete independence from Poland. There could be something of an "reverse Grunwald" with the Polish forces facing alliance of the Teutonic Order, Lithuania, with more Russian contingents than in OTL, and some Tatar contingents followed by an extensive pillaging campaign in the Polish territories by the members of alliance. Unhappiness of his Polish subjects could force Jagello to drop his (and his successors') claims to the Lithuanian throne.
5th, guarantee that the Grand Duchy is inherited by his grandson (at an absence of other male claimants) and to do something to formulate the ...er... "progressive" rules of succession: from father to son rather than from brother to brother. This means prevalence of the Orthodox Church but, with the half-sane rulers, the rights of the Catholic minority in Lithuania proper are not endangered.
As a result, by 1430 you have an empire, biggest in Europe, stretching from the Baltic Sea to, in practical terms, upper Volga River and, with its vassal states, all the way to Ural Mountains and, potentially, all the way to Western Kazakhstan (terminology "Gold", "Blue" and "white" Hordes has multiple interpretations).