Chris McKenna would probably work as a new showrunner, since the show is definitely losing its other two co-showrunners. I listened to a Nerdist Writers Panel podcast that Harmon was on earlier this year (and another that Ganz and McKenna had been on), and Harmon left me with the impression that those two are probably the biggest influences on the show after him. McKenna is already a co-EP, so maybe he could just be promoted to EP and Ganz could be made a co-EP.