I am writing this before I leave for class so it will be short and unresearched as to the salary implications lol.
Garnett retires at the end of the season, comes back to the Celtics as an assistant coach to train the big men.
Pierce resigns on a sweetheart deal to come off the bench for the Celtics.
Draft Andrew Wiggins and Willie Cauley-Stein (of course this means that they are available at the picks we have).
If you go into next season rolling out a line up with those players and the guys we have now, I think you have a pretty decent young team that would probably contend to make the play offs next year and potentially compete the year after (if everyone progresses at a decent rate comparable to their ability.
May have to look to the possibility of trading Bass in order to get everyone minutes on the inside there because I think Humphries would work just as well as Bass starting at either the 4 or 5. Just a thought though.