Henson for minor prospect/picks(s). Let Horford mold him, and watch his salary look even more reasonable next season. This is sort of a Crowder situation, but a little further along... a clearly talented player that hasn't quite gotten the full time opportunity with the team that drafted him. He's only played in one system, and shown the ability to be at least a 12/8/3(blocks) type player with 25-30 minutes. Certainly his length is a weapon if used correctly.
I'd love to still get Middleton, but the price IMO will be too high. Parker makes no sense for clearing cap room.
Henson is an elite shot blocker and good rebounder who could use a change of scenery, and makes Sully pretty much irrelevant (except in case of S&T or taking the QO).
I'd offer a late 1st, or some 2nd rounders... or one of our lesser prospects. Absorb the cap hit and we should still have room for Amir and Jerebko. Let Amir and Jerebko both come off the bench as versatile vets... they would thrive against second units and could get as many minutes as Henson depending.
IT / Rozier
Bradley / Smart / Hunter
Crowder / Brown / Jerebko
Henson / Amir / Mickey
Horford / KO
Maine/stash: DJax, Bentil, Young, Yabusele, Zizic
That team is legitimately 13 players deep, which could be an issue if all healthy, but pretty darn talented at the 8-13 spots.