Kassel.
The new airports "competition" would be Paderborn 90 km, Dortmund and Hannover 170 km , Frankfurt 200 km and Leipzig 250 km, so the catchment area appears to be good. If the destinations are attractive then I cann't see why Kassel wouldn't work for FR. Cochstedt has loads factors of 90 % on the sun routes and thats in the middle of nowhere.