Natural borders? The answer is only natural: the Atlantic to the West, the Indian to the South, the Arctic to the North, and the Pacific to the East. Poland may not into space, but Poland can into the CK2 map.
Ah, but in seriousness, many of Poland's vices and vicissitudes alike could be tied in part to the fact that it lacked natural borders in key directions, and thus its borders in the east and west relied almost entirely on the strength of the state. When strong, it sprawled across everything in its area; when weak, it was sprawled upon. If you want natural borders for a pan-Slavic state, it depends on when you set the game start. In the 8th or 9th century starts, the Elbe is worthwhile to include the remains of the Wends/Obotrites and Sorbs. In 1066, that border's already been pushed back to the Oder-Neisse and beyond, but there may be some merit in rolling it back. The Baltic will likely be your northern anchor, and the Balts, while not Slavic, are small and weak. The southern line might be the Carpathians, which are perhaps the oldest Polish border, but if you want the southern Slavs to join your mighty pan-Slavic state, then the Adriatic and Mediterranean are your best recourse, anchoring your western border either along the Leitha River (roughly between Hungary and Austria) or the a bit of the Austrian Alps up to the Bohemian Massif. Especially in earlier starts, the Carpathian basin was poorly settled and semi-nomadic, seeing multiple migrations from eastern tribes (in game period the Avars, then the Magyars, then the Cumans). East...heh, there's a reason why these borders were particularly fluid. The Daugava and Dneiper are an early natural border, with the gap between the two being a significant part of the reason Smolensk was considered the gateway to the east (or west), and that already puts you pretty far east: Kiev, the first Russian state, is on the Dneiper. Further on, there's the Don and Volga, or the Volga and Dvina, or the Volga, Kama, and Pechora, or at that point, you may as well just run right up to the Ural Mountains and beyond.