I'd probably be willing to take on Johnson's contract, but I'd also want at a minimum Josh Smith (since I know Horford is out of the question).
I'd consider something like this
Boston - Josh Smith, Joe Johnson, Jeff Teague
Atlanta - Ray Allen, Kevin Garnett, Avery Bradley
The Celtics upgrade at every position in the trade, but take on a lot of long term salary. Atlanta essentially becomes a player next summer with just Horford, Williams, Pachulia, and Bradley all they have on the books and still can remain competitive and interesting this season.