Kind of a difficult question, which really depends on your definition of superpower. Though I can list off a few (European) nations you might want to look at.
Scotland - Though once you kick off England you're probably in the same position as your last game. A little tougher doing it, I'd imagine.
Teutonic Order - Technologically advantaged but surrounded by larger nations.
Italian nations - Most of them have at least a few provinces to work with. Naples and Milan are the biggest.
Burgundy - Tough if you're playing In Nomine, France is a killer. Otherwise I've heard they can basically be just as strong as France, so it might not be a good choice if you're looking to avoid superpowers.
Aragon - Not terribly weak but you have to worry about France sooner than Castille.
Brittany - Again, if you're playing In Nomine, you're going to be in for a challenge.
German nations - There's a few modest sized ones mixed in between all the OPMs. Bavaria and Brandenburg come to mind. Though whether you want to be part of the HRE politics, well, that's up to you.
Holland (Or nearby OPMs) - Possibly another choice. Starts really small but can easily become one of the wealthiest nations.
Portugal? - Not exactly large, though some consider it a superpower. Good if you're aiming for colonies.
These are the nations I can think of. Not terribly small, but not really superpowers. They shouldn't be too hard to play.
Now if you're aiming for something non-European, well, I can't make any suggestions there. Though I wouldn't go for an African or New World nation if you're still relatively inexperienced at the game.