Some perhaps realistic options where in most cases the Celtics should ask for additional assets:
LaMelo Ball (Hornets)
De'Aaron Fox (Spurs)
Jamal Murray (Nuggets)
Darius Garland (Clippers)
Trae Young (Wizards)
Donovan Mitchell (Cavaliers)
Devin Booker (Suns)
Kawhi Leonard (Clippers)
Amen Thompson (Rockets)
Paolo Banchero (Magic)
OG Anunoby (Knicks)
Trey Murphy (Pelicans)
Giannis Antetokounmpo (Bucks)
Zion Williamson (Pelicans)
Evan Mobley (Cavaliers)
Lauri Markkanen (Jazz)
Alperen Seng?n (Rockets)
Jalen Duren (Pistons)
Bam Adebayo (Heat)
Domantas Sabonis (Kings)
Current or future stars like f.e. Brunson, Maxey, Cunningham, SGA, Edwards, Knueppel, Doncic, Barnes, Flagg, Wagner, Holmgren, Towns, Wembanyama and Jokic are out of the picture I believe given their status, potential or importance.
As a side note I would love to see Robert Williams back this summer or give Paul Reed the chance to blossom as a solution to strengthen our center position.