With the Celtics now above the tax line and only a few million below the apron, a sign-and-trade is highly unlikely. There’s just not the flexibility without moving a large salary.
Most likely outcome would seem to be they use the 6 million taxpayer exception and call it a day, or close too it. Rumors that Dunn could be moved again as well.
One reasonable scenario I could think of for a S&T for Lonzo would be something based around Smart. If NOLA was able to land Lowry, I could see them being potentially interested in a S&T with Lonzo based around Smart and Romeo/G Williams for around $80M/4 years. That'd be a really interesting lineup for them in Lowry, Smart, Ingram, Zion, and Hayes. They probably compete for homecourt advantage in the playoffs and potentially take a series or two barring health concerns.
For us, it'd kill two birds with one stone - figure out the Smart situation and get our long-term PG that seemingly fits well with the Jays and is on their timeline. Though, the question is whether that move is better than just keeping the space for the possibility of a Beal, Kawhi, etc. next summer.