I think KG retires. I think Boston then either buys out or trades Pierce.
If that happens, I'd like to see Boston move Rondo and go into full bore rebuilding. I looked at a number of trades and the one I think makes the most sense is
Rondo, Green, Melo, 2014 2nd, and 2015 1st (top 5 protected) for Kemba Walker, Tyrus Thomas, Ben Gordon (if he opts in), rights to Detroit 1st, rights to Portland 1st.
Boston then goes into full bore rebuilding with 3 potential lottery picks in the 2014 draft (though the Portland pick is top 12 protected for while and Detroit's is top 8 protected next year).