3-Team trade I think might work
Boston - Harden, McLemore, Clemons, Sumner
Indiana - Walker, Edwards, BOS 2021 2nd, OKC 2021 2nd (from BOS)
Houston - Oladipo, McDermott, Langford, Nesmith, Green, BOS 2021 1st, BOS 2023 1st (lotto), BOS 2025 1st (lotto)
Indiana would obviously have to believe Oladipo isn't going to stay and prefer more long term security in Walker. Houston would probably need to believe Oladipo would stay, but I think that is solid enough value.
So Boston post-trade (with 1 open spot)
PG - Harden, Teague, Pritchard, Sumner, Clemons
SG - Smart, McLemore
SF - Brown, Ojeleye, G. Williams
Pf - Tatum, R. Williams
C - Thompson, Theis
A little light down low, but with the open roster spot Boston can find someone, and I do think that team could certainly compete for a title the next two year. Certainly far more capable then the team with Walker.