But I did not find an overview what features will come or what features had been implemented before. Is there something like such an overview? Or is the user guide so up to date that all the enabled features where listed there?
As the roadmap wiki site mentioned, we don't have a scheduled roadmap with features but implement features on community request. In the 5.9 release that's coming early March you'll see the addition of compensation, transactions etc functionality based on user request.