Jermaine O'Neal, but only for 1-2 years. Allows us to trade Perk, who won't get any better than he is right now (appears to be a poor man's version of Rick Mahorn, only dumber and with less offense, imo), and Baby for a future first round pick that hopefully ends up in the top five (ala Gerald Henderson for Len Bias) to keep the success going long term.
In addition, JO is a big time shot blocker and defender, likes to play in the low post (
http://www.nba.com/2010/news/features/fran_blinebury/02/17/jermaine.oneal.feature/), doesn't need many minutes or shots to be effective (just look at his stats for this season: 28.7 minutes per game, 10.7 shots per game, 53.3 fg%, 1.4 blocks per game, 13.8 points per game, 7.1 rebounds per game), wants to sign with a contender in the offseason (
http://www.indystar.com/article/20100403/SPORTS04/4030341/1088/SPORTS04/O-Neal-considers-his-next-last-move), can pass, and allows us to attack the centers of Cleveland, Orlando, Los Angeles, Atlanta, Milwaukee, Chicago, etc.
All that's left to do is draft well and play the pick(s), rid ourselves of Tony Allen (god I hate that guy) and Scal (nice guy, but a joke), and acquire Jerry Stackhouse, Raja Bell, Jamaal Tinsley or another veteran point guard BEFORE the season, Jeff Foster, and a power forward (either through the draft or trade/free agency: Omar Samhan, Kenneth Faried, Joe Smith, anyone).
We'd have the best starting five and bench in the game:
Center: JO (26-28 minutes per game)
Power Forward: Kg (28-30 minutes per game)
Small Forward: Paul Pierce (30-34 minutes per game)
Shooting Guard: Ray Allen (28-32) minutes per game)
Point Guard: Rajon Rondo (36 minutes per game)
Bench
Sheed, Foster, Stackhouse, Bell, veteran point guard, draft pick(s).
Hey, I can dream.