Where are the weak parts of my team and who to trade for..?

It would help to know how many teams in the league and all that. From what I can tell, your outfield needs a ton of work, I would try to work on that instead of adding another 3B in Zimmerman. Relying on an often-injured Carlos Beltran as your #1 outfielder and underachieving Alex Gordon as your #3 outfielder are big weaknesses. Don't get me wrong, I like both of them, but you need a solid top 5 or top 10 outfielder leading the way.