There are many systems that provide users with an electronic identity (eID) to sign documents or authenticate to online services (e.g. governmental eIDs, OpenID). However, current solutions lack in providing proper techniques to use them as regular ID cards that digitally authenticate their holders to another physical person in the real world. We envision a fully mobile eID which provides such functionality in a privacy-preserving manner, fulfills requirements for governmental identities with high security demands (such as driving licenses, or passports) and can be used in the private domain (e.g. as loyalty cards). In this paper, we present potential use cases for such a flexible and privacy-preserving mobile eID and discuss the concept of privacy-preserving attribute queries. Furthermore, we formalize necessary functional, mobile, security, and privacy requirements, and present a brief overview of potential techniques to cover all of them.