I think Sefosha would be a good guess, but he can't stay healthy. Fits the profile, 3 pt shooter, veteran.
Nwaba maybe if Ainge wants another perimeter defender?
Tyson Chandler doesn't appear to have much left, can't imagine he'd be a target. Same for Gasol, Burke, Faried, etc. I don't see any big man out there that appears he could give us much of anything.