Sin: Would unified theming (something like qtcurve) belong here? How about another high-level service, a web browser element (webkit) that can be used from any application?
Elektrolott: How about using Akonadi?
Akonadi - The PIM Storage Service Mission Statement We intend to design an extensible cross-desktop storage service for PIM data and meta data providing concurrent read, write, and query access. It will provide unique desktop wide object identification and retrieval.
PolicyKit concerns authorisation of priviledged actions requested by an unprivileged user. This is independant of the actual functionality and APIs. Using PolicyKit may make sense later, when the functionality has been finished.