The primary problem I have with Poland controlling enough territory is less with actually taking the territory as it is holding onto it, at least from what I can see. Poland is, as has been said, stuck between some very powerful nations. Poland will probably need to hold on to Muscovy to...
AHC: Polish Superpower/Hyperpower Status
The Challenge? Put Poland into a position where it can become a global superpower by 2050, using a PoD at or after 1025 CE.
Hard Mode: Poland becomes a superpower by 1980
Very Hard: Poland becomes a superpower by 1980, but, the Second World War...