I proposed this type of trade awhile ago, and I really think something like this is the only way Thomas gets traded
Boston - Wall, Gortat
Washington - Johnson, Jerekbo, Young, Hernangomez, Mudiay, 2018 Boston 1st (top five protected), Memphis 1st, 2017 Denver 1st (top ten protected)
Denver - Thomas
Washington goes into full on rebuilding by acquiring Mudiay, Hernangomez, Yabu or Zizic, and a few 1sts and gets out from under Gortat's contract. Denver goes into win now mode by acquiring Thomas. Boston upgrades the PG and C positions to try and build a real contender (and still keeps the Brooklyn picks though 2018 could only be traded on draft day).
So post-trade Boston looks like this (with 2 open roster spots)
PG - Wall, Rozier, Jackson
SG - Bradley, Smart, Green
SF - Crowder, Brown
PF - Horford, Olynyk, Mickey
C - Gortat, Zeller
I don't think Washington is going to trade Wall, but if they do I think that is a reasonable trade for them depending on what they think of Mudiay. I think Denver might do this as they are on the cusp of the playoffs and they may think acquiring Thomas gets them there. Boston takes a big step closer to Cleveland and should move ahead of Toronto for the 2nd best team in the East.