Based off of history, the trade package I could see includes either:
1 mid-range first rounder + a role player/prospect
or
High first rounder (there are some interesting historical trades that would compare to this)
This year is a bit unique in the draft, with the top 4 of the top 10 teams having deep playoff run aspirations for next year (Mavs, Spurs, Sixers, Rockets), plus multiple other teams that seem poised to try to take the next step (Hornets, Raptors, Pelicans).
The cleanest iteration of the trade I could see is with Holiday going to one of those teams who want to acquire a guy that could help them to take the leap, with the Celtics receiving their 1st rounder, and the Nets receiving the Celtics 28th pick with the salaries from that team to make the trade work.
This may not get the Celtics all the way under the 2nd apron, especially if they use the injury exception from Tatum's injury, but it would get them close. Plus, it allows them to invest a high pick in young talent, which is something we haven't done since we drafted Tatum.