The Bengals have two receivers entering the final year of their contracts and the one who probably isn't getting re-signed is Boyd. Cincinnati spent a second-round pick on Boyd in the 2016 draft and he went on to be a star for them in some not-so-great times.
Unfortunately for Boyd, the Bengals added both Chase and Higgins, which diminished his role. A once WR1 for the team, Boyd was relegated to WR3 duties and while he's still a WR2 on most teams, he won't be a Bengal after this year more than likely. That could make him a trade candidate.
As is the case with Higgins, I don't see the Bengals trading Boyd. He's a valuable member of this offense and they'll want him on the team while they try to make a run at a Super Bowl title.
That being said, if a team suffered an injury at wide receiver and the return haul was decent, it wouldn't be the craziest thing in the world for Cincinnati trade Boyd. They're probably not going to re-sign given the other contracts they have coming up to address so if they can get something in return for him, it could make sense.
I want to reiterate that I don't think the Cincinnati Bengals will make any of these trades but if any noteworthy players were to be traded, these would be the names.