I honestly wouldn't mind any of the three in the right trade and I actually think a Rondo/Williams backcourt would pose all kinds of problems for opposing teams (Williams for all his flaws is still an excellent outside shooter hitting over 40% this year on over 4 shots a game).
I think something like this might make sense for all 3 teams
Boston - Lopez, Williams
Detroit - Thornton
Brooklyn - Smith, Wallace, Bass, Faverani, Powell, Detroit 2016 1st (protected), LAC 2015 1st, lesser of BOS/BRK 2016
Detroit unloads Smith at the cost of a future protected pick.
Brooklyn replenishes the draft picks a bit and at least gets a name back in Smith while shaving future salary in the process.
And Boston in my mind has a real shot if Lopez, Williams, and Rondo are all healthy come playoff time and it fixes some of the issues the team has (Williams has that #1 scorer mentality and Lopez is a legit center) and doesn't cost Boston much (a couple of later 1sts and cap space).
PG - Rondo, Smart, Williams
SG - Williams, Bradley, Turner
SF - Green, Turner, Young
PF - Sullinger, Olynyk
C - Lopez, Zeller