Sixt and Europcar are the big rental companies. Avis and Budget to a lesser extent. At the Frankfurt or Munich airports you can rent just about anything cool and fast. You just have to be over 25 years old and have a cc.
https://www.sixt.com/car-rental/Germany
https://www.europcar.com/location/germany
https://www.avis.com/en/locations/de
https://www.budget.com/en/home
http://www.autoeurope.eu/luxury.cfm