Some ideas for discussion (not saying I like any of them, just possibilities):
Myles Turner + Protected First
Marcus Morris
Porzingis for Fournier + Thompson
McCollum for Fournier + RWilliams + Pick
Kuzma + Something
Dejounte Murray
Rubio
Not sure any of these are realistic proving the most likely outcome is he is on the Celtics next season. Porzingis is an interesting option though.
Myles Turner = I do not see the value in him with Horford back. They play the same role. Horford would have to be traded elsewhere to make it worthwhile acquiring Myles Turner. I'd rather keep Fournier.
Marcus Morris = selfish gunner. Glad to see the back of him. Do not want him here. He destroys ball movement. He is the opposite of the type of player this team needs.
Porzingis = yeah, gotta take that risk. His injury proneness is scary but he is soooo much more talented than what we are giving up. Gotta risk that.
McCollum = hell yes, that is home run. That would be stealing from Portland to get such a talented player at such a low price. Celtics immediately become a title contender. Reminds me of the Lakers getting Pau Gasol from Memphis at a bargain basement price.
Kuzma = meh. Fournier is better.
Dejounte Murray = definitely. Murray is a young guy with upside and is already better than Fournier. That would be a home run. Another long term piece to put alongside Tatum and Jaylen unlike Fournier who is just a medium term fit.
Rubio = nah. Bad fit here. Lack of shooting.