It seems to me like Carter is an ideal frontcourt partner with Banchero, Wagner, and players like Okeke. He's more 5 than 4, and still struggles defending the pick-and-roll, but he's a solid young player that can develop with their core.
The guy I'd love to get is Isaac. He's an ideal buy-low candidate right now with upside as a game-changer on defense. The risk is not great either, because his contract has increasing non-guarantees that make it moveable and or waiveable if he doesn't work out for us.
He's unselfish, doesn't need the ball alot, and skilled. He's also got a little dog in him. Check out his highlights of him going toe-to-toe with Giannis.
In a trade like that, the Cs would need to send Theis as well, who I think makes a lot of sense as a veteran big man behind Carter. That way, they can keep Wagner and Banchero from being beat up by opposing centers.
If you use the Jerami Grant trade as a baseline, then the Cs would need to send out a first rounder and swap some seconds. Grant is on an expiring, but he was also more proven and less injury-ridden. So I think this cancels out.
On top of that, they could send Nesmith and take Ross into their TPE. Ross makes more sense for a playoff team and Nesmith could get minutes on the wing to develop as a shooter.
Celtics: Isaac, Ross
Magic: Theis, future 1st, Nesmith, Stauskas
Celtics lineup
PG: Smart, White, Pritchard
SG: Brown, Ross
SF: Tatum
PF: Horford, Isaac
C: Williams
After that, I'd be looking for a bit more veteran size with the tax payer MLE/minimum contracts. PJ Tucker, Thaddeus Young, Kyle Anderson, Nic Batum, Mo Bamba, Javale McGee, Robin Lopez, Bismack Biyumbo, Luke Kornet, etc.
I'm also holding out hope that Hauser and Fitts can find a role off the bench on the wing.