[a] A lack of quality assets to put alongside Horford in a trade is an issue. There is no young talent to pair him with. Nesmith, Romeo and Pritchard have all failed to show themselves deserving of major roles on a team thus far.
{b} Rob Williams is a possibility but (1) Stevens has him as close to untouchable (2) many teams around the league are wary of giving up big trade returns for low skilled centers these days as much of their contributions can be replaced by cheaper alternatives.
[c] The third issue I would raise along with the lack of youth and the Rob Williams situation is the consequent forced reliance on giving up future draft picks in order to get a low quality star or perhaps multiple picks and swaps in order to get an aging highly paid quality star on the downward end of his career.
To do this would require Stevens to go all-in or heavy-in in a trade since he would be conceding what little flexibility the team has left to improve itself. It is a big risk and requires a front office leader who is comfortable with big risks. It remains to be seen whether Stevens is much of a risk taker or not. The lack of rumours suggests he is not.
You keep saying Robert Williams is low skilled and the truth of the matter is you are dead wrong. If Timelord develops a reliable outside shot, one that goes out to 15-23 feet, he then has a multitude of skills. Given he has risen his free throw shooting into the low 70's there is great reason and hope to believe he can develop that shot
He is one of the best shot blockers in the league. He can defend at high levels down low, both one on one and from the weak side, at a tremendously high level. He has the quickness and length to switch down to guard and defend them at the perimeter. He most likely will be on several All-Defense teams.
On offense, he is one of the best pick and roll/lob finishers in the league. He sets great picks while barely getting offensive calls against him. His passing skill and ability to see the court and how it should work is well above average, if barely used. He has become a credible free throw shooter. He has decent dribbling skills, which simply aren't used at all and is an amazing rebounder.
Get Robert's shot developed outside so that he can hit it when sagged off or used in the pick and pop and then utilize him as a top of the key offensive initiator, and I am not sure how many more skills you would want out of a young player.
Get his outside shot developed then, begin to use his inherent other skills that are definitely there but underutilized, and you have a complete player that can play the 5 and switch down considerably on defense.