Gordon Hayward, 14, 26, and 30 to Atlanta for Collins, 6, and a 34 million dollar trade exemption (Hayward's salary). This seems unlikely to me, but it was an idea I came up with.
Atlanta seems like they are ready to try to win now. The trades for Capella and Dedmon definitely communicate that. But they need another guy on the wing. They could target someone in free agency instead, but if they really want to get into the playoffs, a player like Hayward would be a perfect fit in their offensive scheme. On top of that, they can keep their cap space for 2021 and try to make a run at Giannis. This trade really only makes sense if they look at all their high draft picks over the last few years and think, "Man, maybe it would be better to get proven talent instead of drafting someone early this year." Then, they could take a few swings with their later draft picks and try to get a rotation guy.
The Cs reportedly were talking with the Hawks about Collins at the trade deadline. This trade cleans up the big salaries, gives them a good young offensive big man, and gives them a swing at 6 (I'd draft Hayes if available). On top of that, they have a huge trade exemption to use over the next calendar year on another rotation piece.
Hawks: Young-Heurter-Hayward-Hunter-Capella
Celtics: Walker-Smart-Brown-Tatum-Collins