I'd take Barnes off of Sac's hand anytime. Career avg of 14/5, played on championship teams in GS, has played 6th man before and started.
PG: Kemba Walker - Jeff Teague - Pritchard
SG: Jaylen Brown - Marcus Smart - Romeo Langford
SF: Jayson Tatum - Aaron Nesmith
PF: Harrison Barnes - Grant Williams - Semi Ojeleye
C: Tristan Thompson - Daniel Theis - Robert Williams
Still need to fill out the roster, Believe we have two TPE's from Porier and Kanter, a BAE, and if it's Barnes, the rest of that TPE. Can't combine them, but still should be able to fill in the roster nicely.
Would love Barnes. Would be one of those moves that gets looked back on as the final push of a championship team.
Other players I think would be of huge help to the team in that backup sf/pf role would be:
Terrence Ross
Norman Bjelica
Aaron Gordon
James Johnson
Taurean Prince
Rudy Gay
a Healthy Justice Winslow
Norman Powell
GR3
Perhaps Donnovan Mitchell will just demand he get traded here. He will fit into the tpe easily since his extension hasnt kicked in yet. 
Harrison Barnes = Jeff Green. He's a fine rotation piece to have but I don't think he adds a lot to this team. His defense has gotten a bit worse in Sacramento on and off ball. If he were an expiring contract that would add no financial obligations past this year, I think I'd be more for it. But if he is going to add complications like needing to trade him away or picking between him and someone else in coming years, I'd rather not.
I like the idea of Terrance Ross. He's a guy you can play in the playoffs and can absolutely swing a game for you. I'm out of Bjelica. He can't play defense in the NBA.
Of the other guys you listed, many of them either duplicate too much of what our team already does or have those extra years that make our cap structure pretty difficult moving forward.
The one I would do that for is Aaron Gordon, if the Magic would consider some combination of Robert Williams or Romeo Langford with a future first. I think Gordon could play small ball 5 and would be a solid defensive presence to use in spurts against Bam, Giannis, Durant, etc. His one-on-one defense against the best players last year was sneaky good. I also love the idea of making him the screener in the pick-and-roll game with Walker.
aaron gordon is an excellent athlete and a good basketball player. but, i am not sure he fits with the celtics very well.
first, he is making almost $20 million, which would put a strain on boston's budget to keep him beyond this year.
next, he is pretty poor shooter from mid and long range, only at 32% lifetime on 3 pointers. this means he cannot really be on the floor at the same time as TL, theis, or thompson without hurting the team's ability to stretch the floor.
so, that is a lot of money to pay someone who will see situational minutes.
third, gordon is 6'8". that means he likely will not solve the challenge of a celtic center who can really slow down the big centers in the league. as a center, he would be 3rd or 4th i think on the roster.
i like gordon, but i dont see him fitting well with the celtics.
further, Orlando turned down an offer of two first round picks for Gordon. so any offer would have to start there. that is a lot to give up for him.
if ainge uses the TPE on a mid-season trade, i could see rose depending upon walker's health and teague's performance. larry nance might be a fit as a wing.
i think ainge will wait and see whether there is an injury or need, then decide.
