We could use a star upgrade at every position but in terms of greatest need, a versatile F would really be huge for this team. Enough size to guard PFs and occasionally Cs but with the speed to capably guard SFs. On offense, someone who can shoot the 3 but also drive to the basket and finish inside. Of course, the way the league is going every team could say they need a player like that.
There really are only a few guys who fit that description to a T and they play for the best teams. Leonard in SA, James in CLE, George in IND, and Green in GS. A poor man's version would be Millsap in ATL.
I love Crowder but he's too small to really play PF for an extensive period of time, along with the fact that he's not really a playmaker. A lineup of Thomas, Bradley, Crowder, AWESOME THEORETICAL PF GUY, and Johnson would be really killer. You'd have four 3 pt. shooters on the court and "good enough" rim protection without sacrificing overall team speed. It wouldn't be a great rebounding team unless theoretical PF guy was a great rebounder, but that's also where you could look into upgrading Johnson.