For Carolina, the move makes a lot of sense: Mayfield has played poorly this season, and he's the third-best quarterback on the roster behind Sam Darnold and PJ Walker. Trading for him this offseason proved to be a mistake.
I think the likeliest suitor clearly are the San Francisco 49ers, who just lost Jimmy Garoppolo for the season. Kyle Shanahan might think he could get Mayfield up to speed quickly enough that Mayfield could perhaps be a better option than Brock Purdy as the 49ers make a push to the playoffs.
Mayfield has played in 372 of the 705 offensive snaps in Carolina this year, according to Pro Football Reference.