LeBron
Durant
Harden
Griffin
M. Gasol
Howard
That's the list of guys who would definitely make us a contender if we were just getting them, assuming a competent and complementary supporting cast. Obviously I'm leaving out point guards like Paul, Parker, Westbrook, and Curry, because they wouldn't fit with Rondo.
As others have said, there's another tier of players that might be enough if we got 2-3 of them to add to Rondo.
In any case, I think it's fair to say nobody in the first group is likely to be available for trade anytime soon -- certainly not for what we have to offer, sans Rondo. Some players in that second tier will probably be available, but acquiring not just one but two or even three of them might be nearly as difficult as getting one player from that first group.