I think the trade to be made is Walker and basically every draft pick unprotected that Boston can trade. Other parts and pieces can be included. I think you'd probably try to find a 3rd team to take Walker since he doesn't fit into a rebuilding team (which Washington would be making the trade).
I think something like this may work.
Boston - Beal, Burks, Bullock
New York - Walker, G. Williams
Washington - Ntilikina, Noel, Langord, BOS 1st's (21, 23, 25, 27)
I think something like that makes sense for all 3 teams.
Boston would then be
Starters - Smart, Beal, Brown, Tatum, Theis
Playoff Rotation - Teague, Burks, R. Williams, Thompson
Reg Season Rotation - Pritchard, Bullock, Ojeleye, Nesmith
Suits - Green, Edwards, Waters, Fall
I don't know if that team competes with the Nets, Lakers, etc. this season, but 2 years down the line, I think that would be a championship level team (for at least a few seasons) if Tatum and Brown continue to progress like they should and Beal doesn't fall off a cliff.