The Cincinnati Comic Expo is Sept. 20, 21 and 22 at Duke Energy Convention Center.
http://cincinnaticomicexpo.com/
The guests I've heard of are Jodi Benson, Kathy Najimy, Nichelle Nichols, Brent Spiner and Wil Wheaton.
This same weekend, downtown, is Oktoberfest Zinzinnati. I would advise not going downtown!