I have played many games as Prussia starting in 1520. They are a minor and only have 4 provinces. If you can avoid war with poland, and diplomatically annex a couple german minors, pretty soon you have enough income and good enough leadership to make an impact on european affairs.
You love editing your game files don't you, Prussia was a fief of Poland at this time

However yes, Prussia is a very interesting nation to play. When I get time I'd like to vassalize most of the HRE and create a united German super-power, rivalling France to the west and Poland to the east. It would certainly make THAT particular game, very, very interesting. What would make it better is seeing the a.i aquire most of the HRE itself and then see what it can do with it. A whole new dynamic would be make its mark on Europe!
The closest I've seen this occur is the principality of Bavaria vassalizing over 70% of the HRE, but this was never transferred into peaceful annexation, which I found very disappointing because the Bavarians managed to vassalize them over a 100 years before the end of the game.
Sapura