I would guess that if SAS trades Leonard they are going to want to rebuild, so they are going to want as many young players and assets as possible, so Tatum, Brown, Yab, Rozier, LAL/SAC 1st for Leonard (I think that works using next years' salaries - if the pick conveys this year, we can probably keep Yab and Rozier from a value standpoint). It is a lot to give up, but I think that makes sense for both teams. So Boston post-trade (assuming pick doesn't convey)
PG - Irving, Smart (re-signed)
SG - Hayward
SF - Leonard, Nader
PF - Morris, Theis, Semi
C - Horford, Monroe (re-signed)
Add Boston's 1st this year and some vets to fill in the bench.
That team is a clear contender and will be one for at least 5 years (assuming health).