I can't emphasize enough that I really don't want to trade Hayward, but as a thought exercise...
My favorite idea is a three-team trade with the Nuggets and Wolves.
Boston receives: KAT, Beasley
Denver receives: Hayward, Covington
Wolves receive: Milsap, Porter Jr., Robert Williams, Langford, 2020 Memphis pick, 2020 Celtics pick, 2022 Celtics pick
Wolves get three exciting, high upside prospects and a host of future pics to pair with Culver and Wiggins moving forward. Their coach can form the team's fundamentals with them as young players and build the Wolves from the ground up.
Denver gets much, much better. Hayward would play off of Jokic so, so very well. Covington spreads the floor and gives them a rangy defender to go against Leonard, George, and James. Jokic-Grant-Hayward-Covington-Murray with Barton, Harris, and Plumlee is a really, really good top 8.
Celtics get KAT. Beasley is a good rotation guy depending on what kind of contract he thinks he deserves in the off-season. Kat-Tatum-Brown-Smart-Walker with Theis, Kanter, Williams, Beasley, Wanamaker is a good top 10. Ideally, they could get one more veteran for playoff minutes like Andre Igoudala.
This also is a good move for Hayward, sending him to a contending team in the west where he can thrive and where he will likely get a max extension offer (Denver does that with their players).