Here is a possibility I see: the Intermarium is formed following amazing success by the Pilsudski adminsitration, it embraces democratic reform to stave off independence movements, Dmowski's followers are spurned and exiled, multiculturalism is embraced, as is the practice of investing massive amounts of public funds into militarization. There would still probably have to be crackdowns against dissent at some point, but native forces would have to decide to hold on to the fragile confederal structure that would develop. Essentially, this would be a European Union of eastern Europe united by Pilsudski's iron-will and successful collaboration with disparate parties and organizations. I imagine there would be widescale deportations of Russians to the Soviet Union and Jews to Israel as the federal government pursues a populist-authoritarian model similar to fascism. They manage to put together a nuclear program with a warhead in the works by 1944, and for some reason their military is large enough that the Nazi Germans and Soviet Union decide not to attack, fearing a long war that would open them up to attack by the Western powers. Instead, the Soviet Union launches itself into a war for China against Japan. By 1946, with the invasion of Sudetenland and the Polish corridor, the secession of Finland, and the invasion of Ukraine ongoing after the annexation of China and the nations of Indochina, the Intermarium stuns the world by detonating a nuclear weapon over a major Konigsberg military base. It is followed with an entire German army obliterated by a timed-detonation of a series of nuclear mines, a tactic that is replicated with similar great effect against the clueless Russian commanders that had reached Romania in a blitzkrieg assault. The war is ended with a promise to return to the status quo ante bellum. Soviet soldiers withdraw en masse from Belarus back into Russia proper.
The Intermarium Federation is maintained and experiences further democratic reform, as well as the development of its own eastern european culture. The Nazis endure until the 1960s, when they are toppled by popular protests following the death of Hitler. The Soviet Union develops nuclear weapons, as does the United States and the Western powers. The Cold War takes place with the Intermarium being the doorstep to the West, and China remains closed to the world. Japan transitions from an empire into a modernized constitutional monarchy. Eventually, in the 1980s a new generation decides to topple the aging communist governments. China becomes independent, finally, as does Korea and the Indochinese countries. The new millennium starts amid a spring of nations across Eurasiona.