Well, the obvious answer is a good center of course.
Point guard is a lock, there's no need to change anything there.
I am also content with our general rotation at the 2, but if I could choose one guy in the league currently, I'd go for Bradley Beal.
At the SF position, you could argue it would be pretty hard to upgrade. I think that Green, with all his flaws with inconsistency, is a top 7 at his position. But if I could choose one guy outside of KD and Lebron, it would be George.
At the PF/C position, there are 4 guys I would get without a blink of an eye. Gasol, Boogie, Davis and a healthy Horford.
At what expense? Anyone but Rondo, Sully and Smart. Although I could learn to live without the latter two for an Anthony Davis or Boogie Cousins.
So, after all this, my dream lineup for contention would be Rondo, Beal, George, Sully, Gasol. But I would be pretty content with Gay at the 3, or Green, if he somehow decides to become consistent and aggressive every night.