For speed-kini.de I extended the standard user system with a custom registration form. GravityForms offers a simple add-on which can provide custom registration and profile update pages (http://www.gravityforms.com/add-ons/user-registration/). The custom field values can be easily accessed in PHP later.
REST API extension
In order to provide dynamic front-end functionality, I extended the built-in REST adapter for my specific purposes. I started with JSON data about riders which is displayed in their user profiles. There is a difference between registered users who a WordPress user profile and people who are only identified by their names. There are different REST paths for both possibilities.
Frontend Ajax and Google Charts