Here's an idea:
Celtics get: McCullom, Grant, McGruder, out of luxury tax
This trade is admittedly unlikely for the Cs, because it gets us the two best players in the trade and we get out of the luxury tax for one more year. A lineup of McCullom-Brown-Tatum-Grant-Williams would have a high offensive and defensive floor.
Blazers get: Pritchard, Horford, 1st round pick (any year of their choosing), out of luxury tax
As the Blazers look to hit the reset/rebuild button, getting out of McCullom's contract while getting good value back will be important. This value is based on previous reports that McCullom is viewed as a negative value contract on his deal (I find this hard to believe). Here, they get hometown favorite Pritchard, a first round pick, Horford's partially guaranteed contract, and they get out of the luxury tax.
Pistons get: Smart, Richardson, Mclemore
Smart just seems like a Detroit Piston and seems like the kind of player that Dwane Casey would love and develop. Pairing Smart with Cunningham makes good sense on paper as well. Richardson also seems like a Casey guy. These seems like really good value for Grant, almost too much.
I wouldn't mind sending Langford or Nesmith as part of this trade either, but it does complicate things with the luxury tax.