I would keep Amir Johnson and Jonas Jerebko at least another year. Even with Horford we need some size, and both players definitely provide that temporarily. Jerebko is also versatile and at 5M/Year, a bargain.
Let Zeller and Sullinger walk. Zeller was practically a non-factor last year and Sully likely gets 18-20M/year, no thanks.
Terrence Jones is an intriguing option. Young, athletic, can rebound. Might come cheap as no teams seem to be that interested in him which is surprising.
Oh and use MLE on someone. Mickey, Brown and Smart likely get more playing time with Turner, Sully, and Zeller leaving.