I have a ton of unrealistic dream trades that are offseason options, because I don't think Beal is getting traded until this summer. That said, here is one I'd be fine with if Danny did it tomorrow:
Celtics In: Harrison Barnes
Celtics Out: Daniel Theis, Aaron Nesmith, Carsen Edwards
Mavericks In: Daniel Theis
Mavericks Out: Josh Green
Kings In: Aaron Nesmith, Carsen Edwards, Josh Green
Kings Out: Harrison Barnes
Rationale: The Thompson-Theis starting duo doesn't seem that successful. With Rob Williams finally looking viable as at least a reserve, the frontcourt is crowded with three players who don't compliment each other when playing together, and one needs to be moved. While I'd prefer to move Thompson, I think Theis is more easily traded, both due to his easier fit on teams and his cheap expiring deal. Barnes could either start at the 4 or come off the bench if Smart remains a starter when he returns, increasing the versatility of some lineups. Presumably a Walker-Smart-Brown-Tatum-Barnes lineup would be given some chances to work as well.
Meanwhile, the Mavericks acquire someone who can play next to Porzingis in 5-out lineups as they try to salvage their season, and they may be interested in retaining Theis next year with his Bird rights. The Kings get two rookie 1st-round picks as they embark on yet another rebuild. Edwards is more of a throw-away -- I think he's a better option than the Kings current last man on the roster (Justin James) -- the benefit of moving Edwards to the Celtics is to create some additional luxury tax space with Brown's incentives potentially kicking in, and the Kings probably wouldn't say no to his inclusion.
The link below shows that the trade works. TradeNBA hates Harrison Barnes and loves Theis, so the Mavs certainly do this deal if that website is correct, and it makes sense for the Kings as well in my book. I think the fit of Barnes with our other bigs is better than Theis, so I disagree about their wins expectation for Boston. I'd strongly prefer to do some deal like this with Thompson moved instead of Theis, but I don't think such a deal realistically exists. (I tried to concoct one with Phoenix getting Thompson, but I can't justify them giving up comparable value for a backup to Ayton at MLE money for the next two seasons.)
https://tradenba.com/trades/vsXmkOyCo