Melo isn't the answer for this team. I still think he has a few deep playoff runs in him, but he needs to go to team that is one piece away from contending. Chicago and Miami make the most sense.
Paul George isn't going anyway, no matter how bad the Pacers are this year.
Demarcus Cousins is gonna be in Sacramento for at least another season. I don't think Vlade or Vivek have any intention of trading him for a bunch of nickels and dimes, and it appears that the Kings will probably be halfway decent this season. Earliest I'd expect DMC to get shopped, no matter how difficult he may be at times, would be the trade deadline in 2017.
I think the best way forward for the Celts will be via the draft. Spend this season figuring out what role players can fit the system and thrive here for the next 4-5 years, continue building up Marcus Smart as the emotional leader and primary ballhandler, and then do whatever it takes to get at least one main building block in the draft over the next 2-3 seasons.
Free agency might offer an opportunity to add a core piece, as well, but the draft is going to have to be a big part of it.
I'm not seeing a Kevin Garnett / James Harden trade coming along to magically fix everything overnight.