According to Chris Mannix, Brown and Tatum are off-limits, as is a future first-round pick.
From CSNNE (starting at 2:45):
Where does this end? Does the deal get called off? Do the Celtics acquiesce and throw in a sweetener?
Mannix: “Well, it depends on how sweet that sweetener is. From what I’m being told, Boston is going to draw a pretty definitive line in the sand.
“They’re not going to give up one of the two young players: [Jaylen] Brown and [Jayson] Tatum. That’s ridiculous. […]
“They’re also not going to give up another first-round pick. They’re done giving up first-round picks here.
“Now Boston, they may throw in a second-rounder. That may be the type of face-saving move that will allow Dan Gilbert to declare victory after all of this.”