Numbers Wizard, Albert Nahmad, appears to have solid respect around the League, and he has indicated that even with the eventual signing of Kemba Walker, the return of Al Horford would be an extreme long shot - but not totally impossible. Even so, the questions to be answered are, would Al consider coming back now that Kemba Walker seems all but locked up - and would Danny Ainge want to commit an extended contract to Horford.
This would free up a lot of possibilities for the Celtics to build out a roster, but would require cooperation from A LOT of parties with different objectives.