Currently, Title and Description properties are typed to string and the control template uses TextBlock controls to display these properties, I think that only typing these properties to "object", showing them with  ContentPresenters and adding a pair of DataTemplate Dependency properties (TitleTemplate and DescriptionTemplate) would offer an extremely flexibility to this useful control.

Declined: If the users need to transfer object data to the ScreenTip they could use the Tag property(http://msdn.microsoft.com/en-us/library/system.windows.frameworkelement.tag%28v=vs.110%29.aspx)  and bind its value to an custom Implicit style targeting the ScreenTip.